Will Outlook.com Calendar Sync ever work reliably

prgeno · July 8, 2025, 9:49pm

I’ve read all the current threads and tried all the suggestions to make this work properly and reliably, yet it simply doesn’t.

I’ve been using eM Client a long time, dating back to 7.xxx and currently on 10.3.2412

The Airsync operation with the Outlook.com calendar works sometimes for reasonably decent stints, but those have become less and less lately, and typically fails at the most unfortunate moments.

When it fails it sometimes pops an error stating “em client [AirSync] Synchronizing folder ‘[email protected]/Calendar’ failed due to the following error: Exception of type ‘MailClient.Protocols.Common.HttpResponseException’ was thrown.”

But other times the calendar simply goes blank and/or has some appointments but adds, moves and changes don’t show up on my other devices (meaning it isn’t uploading/syncing new data).

I obviously like eM Client better than anything else, having been such a long time user. But this has become beyond frustrating. The calendar sync function is just too unreliable which makes it completely useless for something as important as a calendar. It has gotten SO BAD that I’ve had to resort to using Outlook again.

Hence my question: Will Outlook.com Calendar Sync ever get fixed?

cyberzork · July 8, 2025, 10:25pm

Try the latest version just released V10.3.2545 which now sends messages again via SMTP instead of AirSync so might also help with your issue with the Calendar AirSync problem.

https://www.emclient.com/release-history

eM Client 10.3 for PC & Mac Update

Added search when selecting QuickText
Messages are now sent via SMTP instead of AirSync on Outlook.com once again
Fixed importing of some specific messages from PST files
Fixed several issues with message rendering on dark themes
Fixed a few issues with spell checking
A few other fixes

prgeno · July 8, 2025, 10:58pm

Interesting Outlook Calendar wasn’t mentioned, only messages and I don’t think Calendar uses SMTP on the MS side. But release notes are typically incomplete and certainly worth a try.

I like to keep eM Client up to date, and it’s funny the update function in my installed version thinks I’m up to date. But I’ll manually download and install this update to give it a try, even though I’m skeptical. I’ll report back once I install it and give it some time to prove itself for awhile under normal use conditions.

cyberzork · July 9, 2025, 2:01am

When it fails it sometimes pops an error stating “em client [AirSync] Synchronizing folder ‘[email protected]/Calendar’ failed due to the following error: Exception of type ‘MailClient.Protocols.Common.HttpResponseException’ was thrown.”

I personally carn’t replicate that specific (Response Exception was thrown error) with my Outlook.com eg: IMAP account using eM Client on Windows 11 or Mac Sequoia, but I have seen it happen with another user on this forum.

So I can only suggest next, if you still get the same error with the latest V10.3.2545 version try removing and readding your Outlook account via “Menu / Accounts” using the automatic account wizard and if that fixes the problem.

Note: If you do try removing and readding your Outlook account make a manual backup first via “Menu/ Backup” incase you need to restore for any reason. You can see when the backup is complete in Show Operations via clicking the drop-down on the right of Refresh top left.

If removing and readding your Outlook account made no difference, then if you have a current active paid Pro or Personal version, suggest to go to the VIP support page and login and lodge a support ticket to be investigated.

prgeno · July 10, 2025, 12:47pm

Things seem to have settled down the past 24-36 hours. I will keep monitoring to see how things go over the long term, but I’m somewhat hopeful I can stop using Outlook and go back to eM Client full time again, for now.

Michal_Burger · July 10, 2025, 3:39pm

There was no change with Outlook.com calendar which is still based on Airsync protocol (there is no other way how to access it). The problem is that Airsync servers are often failing or not responding, but having logs is always helpful for diagnosing the issues.

prgeno · September 25, 2025, 6:28pm

It sure would be nice if this issue was fixed, or at least remove this function from the eM Client official feature set list since it appears that this will never get fixed.

Michal_Burger · September 26, 2025, 2:17pm

We cannot fix Microsoft outages if yoiu are talking about them. There are no details about your specific issue.

prgeno · September 26, 2025, 2:34pm

The details about my specific issue is in my original post as the first message of this thread.

Calendar AirSync has been unreliable for quite awhile. The claim about not being able to fix a Microsoft outage is understandable but is definitely not the issue as Outlook always works, while eM Client works only occasionally, blanking out my calendar more times than not. eM Client is simply not a reliable AirSync client.

Calendar and Contact entries usually don’t sync until one of the brief instances when eM Client AitSync actually does what it’s supposed to do, sync, but it usually doesn’t.

Michal_Burger · September 26, 2025, 4:11pm

But Outlook use a different protocol to connect to Outlook. You can’t compare that. Are you talking about New Outlook or Classic Outlook?

prgeno · September 26, 2025, 4:38pm

Both

And all my iDevices work flawlessly, and every other email client I’ve tried. eM Client, running on two difference Windows 11 machines, is the only one I have trouble with using the Calendar and Contact sync with Outlook.com

Worse, eM Client clears all the Calendar items before it tries to sync and just spins. So I lose everything between failed syncs. That makes it pretty useless for this purpose.

prgeno · September 26, 2025, 5:21pm

Below are the most recent errors, but there are a few others that show up if/when I add something to either the Calendar or Contacts with the hope they would sync with my other devices.

[AirSync] Synchronization error (InvalidSynchronizationKey)

[AirSync] Synchronizing folder ‘[email protected]/Calendar’ failed due to the following error: Exception of type ‘MailClient.Protocols.Common.HttpResponseException’ was thrown.

prgeno · October 3, 2025, 4:16pm

This ongoing problem, syncing calendar and contacts with Outlook.com, is so disappointing.

After using eM Client for so many years, I’m being forced to go back to using Outlook while searching for a working alternative. It pains me to lose eM Client as I’ve enjoying using it but this problem is simply too much to deal with.

First of all, why can’t eM Client get sync working again as it did in the past?

Second, why must eM Client clear the entire calendar before syncing, which it doesn’t reliably do, thereby losing access to everything while it tries (and fails) to sync? At least it doesn’t do this horrible thing with Contacts.

This is so sad for me, but this issue has been going on unaddressed for far too long and has rendered eM Client pretty useless for my purposes at this point.

GNC6 · January 14, 2026, 3:41pm

Same problem here. I am a new user with eM Client and love many of the features, but this is an issue.

Perra · March 4, 2026, 9:23am

Still the same problem. It will never be solved. Works fine with Thunderbird.
[AirSync] Synkronisering av mappen ‘P @ Outlook/Kalender’ misslyckades på grund av följande fel: Exception of type ‘MailClient.Protocols.Common.HttpResponseException’ was thrown.
EM version: 10.4.4867

isaacchua · March 23, 2026, 6:49am

I just started using eM Client and I’m experiencing the same problem. As a software engineer myself, here’s how I understand the problem: something goes wrong with the sync process at some point, which forces a complete re-sync. I have over 16,000 calendar items over 12 years. The probability that something happens to the connection as it tries to re-download the entire database gets exponentially higher the larger it is. It also takes up a ton of data — frankly, it is horrible on mobile connections.

Digging deeper into the logs and the ActiveSync Command protocol, something strange is happening.

A periodic sync starts with a FolderSync. I’m not sure why eM Client does not maintain a SyncKey for FolderSync and always starts with 0.

13:54:18.586|03B|   FolderSync
14:04:17.344|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:17.344|03B|   <FolderSync xmlns="FolderHierarchy:">
14:04:17.344|03B|     <SyncKey>0</SyncKey>
14:04:17.344|03B|   </FolderSync>
14:04:17.344|03B|   
14:04:17.344|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:17.748|03B|   <folderhierarchy:FolderSync xmlns:folderhierarchy="FolderHierarchy:">
14:04:17.748|03B|     <folderhierarchy:Status>1</folderhierarchy:Status>
14:04:17.748|03B|     <folderhierarchy:SyncKey>1</folderhierarchy:SyncKey>
14:04:17.748|03B|     <folderhierarchy:Changes>
[...]
14:04:17.774|03B|     </folderhierarchy:Changes>
14:04:17.774|03B|   </folderhierarchy:FolderSync>

It then immediately does a GetItemEstimate on CollectionId 5 = Calendar. This is fine, but then it does a Sync after. This is strange because according to the spec, the Sync response will tell whether there are new items, therefore the GetItemEstimate should only come after the Sync says there are new things. (Note: There is no response to the Sync in the logs because when there is nothing to sync, the server responses with no content.)

It then does the same for CollectionId 10 = Contacts and CollectionId 62 = Tasks.

14:04:17.774|03B|   GetItemEstimate
14:04:17.775|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:17.775|03B|   <GetItemEstimate xmlns="GetItemEstimate:">
14:04:17.775|03B|     <Collections>
14:04:17.775|03B|       <Collection>
14:04:17.775|03B|         <SyncKey xmlns="AirSync:">1305192833</SyncKey>
14:04:17.775|03B|         <CollectionId>5</CollectionId>
14:04:17.775|03B|       </Collection>
14:04:17.775|03B|     </Collections>
14:04:17.775|03B|   </GetItemEstimate>
14:04:17.775|03B|   
14:04:17.775|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:17.900|03B|   <getitemestimate:GetItemEstimate xmlns:getitemestimate="GetItemEstimate:">
14:04:17.900|03B|     <getitemestimate:Response>
14:04:17.900|03B|       <getitemestimate:Status>1</getitemestimate:Status>
14:04:17.900|03B|       <getitemestimate:Collection>
14:04:17.900|03B|         <getitemestimate:CollectionId>5</getitemestimate:CollectionId>
14:04:17.900|03B|         <getitemestimate:Estimate>0</getitemestimate:Estimate>
14:04:17.900|03B|       </getitemestimate:Collection>
14:04:17.900|03B|     </getitemestimate:Response>
14:04:17.900|03B|   </getitemestimate:GetItemEstimate>
14:04:17.900|03B|   
14:04:17.900|03B|   Sync
14:04:17.900|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:17.900|03B|   <Sync xmlns="AirSync:">
14:04:17.900|03B|     <Collections>
14:04:17.900|03B|       <Collection>
14:04:17.900|03B|         <SyncKey>1305192833</SyncKey>
14:04:17.900|03B|         <CollectionId>5</CollectionId>
14:04:17.900|03B|         <GetChanges />
14:04:17.900|03B|         <WindowSize>50</WindowSize>
14:04:17.900|03B|         <Options>
14:04:17.900|03B|           <BodyPreference xmlns="AirSyncBase:">
14:04:17.900|03B|             <Type>2</Type>
14:04:17.900|03B|           </BodyPreference>
14:04:17.900|03B|           <Conflict>0</Conflict>
14:04:17.900|03B|         </Options>
14:04:17.900|03B|       </Collection>
14:04:17.900|03B|     </Collections>
14:04:17.900|03B|   </Sync>
14:04:17.900|03B|   
14:04:17.901|03B|   GetItemEstimate
14:04:18.064|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:18.065|03B|   <GetItemEstimate xmlns="GetItemEstimate:">
14:04:18.065|03B|     <Collections>
14:04:18.065|03B|       <Collection>
14:04:18.065|03B|         <SyncKey xmlns="AirSync:">395211212</SyncKey>
14:04:18.065|03B|         <CollectionId>10</CollectionId>
14:04:18.065|03B|       </Collection>
14:04:18.065|03B|     </Collections>
14:04:18.065|03B|   </GetItemEstimate>
14:04:18.065|03B|   
14:04:18.065|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:18.184|03B|   <getitemestimate:GetItemEstimate xmlns:getitemestimate="GetItemEstimate:">
14:04:18.184|03B|     <getitemestimate:Response>
14:04:18.184|03B|       <getitemestimate:Status>1</getitemestimate:Status>
14:04:18.184|03B|       <getitemestimate:Collection>
14:04:18.184|03B|         <getitemestimate:CollectionId>10</getitemestimate:CollectionId>
14:04:18.184|03B|         <getitemestimate:Estimate>0</getitemestimate:Estimate>
14:04:18.184|03B|       </getitemestimate:Collection>
14:04:18.184|03B|     </getitemestimate:Response>
14:04:18.184|03B|   </getitemestimate:GetItemEstimate>
14:04:18.184|03B|   
14:04:18.184|03B|   Sync
14:04:18.184|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:18.184|03B|   <Sync xmlns="AirSync:">
14:04:18.185|03B|     <Collections>
14:04:18.185|03B|       <Collection>
14:04:18.185|03B|         <SyncKey>395211212</SyncKey>
14:04:18.185|03B|         <CollectionId>10</CollectionId>
14:04:18.185|03B|         <GetChanges />
14:04:18.185|03B|         <WindowSize>50</WindowSize>
14:04:18.185|03B|         <Options>
14:04:18.185|03B|           <BodyPreference xmlns="AirSyncBase:">
14:04:18.185|03B|             <Type>2</Type>
14:04:18.185|03B|           </BodyPreference>
14:04:18.185|03B|           <Conflict>0</Conflict>
14:04:18.185|03B|         </Options>
14:04:18.185|03B|       </Collection>
14:04:18.185|03B|     </Collections>
14:04:18.185|03B|   </Sync>
14:04:18.185|03B|   
14:04:18.185|03B|   GetItemEstimate
14:04:18.353|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:18.353|03B|   <GetItemEstimate xmlns="GetItemEstimate:">
14:04:18.353|03B|     <Collections>
14:04:18.353|03B|       <Collection>
14:04:18.353|03B|         <SyncKey xmlns="AirSync:">2521180</SyncKey>
14:04:18.353|03B|         <CollectionId>62</CollectionId>
14:04:18.353|03B|       </Collection>
14:04:18.353|03B|     </Collections>
14:04:18.353|03B|   </GetItemEstimate>
14:04:18.353|03B|   
14:04:18.353|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:18.470|03B|   <getitemestimate:GetItemEstimate xmlns:getitemestimate="GetItemEstimate:">
14:04:18.470|03B|     <getitemestimate:Response>
14:04:18.470|03B|       <getitemestimate:Status>1</getitemestimate:Status>
14:04:18.470|03B|       <getitemestimate:Collection>
14:04:18.470|03B|         <getitemestimate:CollectionId>62</getitemestimate:CollectionId>
14:04:18.470|03B|         <getitemestimate:Estimate>0</getitemestimate:Estimate>
14:04:18.470|03B|       </getitemestimate:Collection>
14:04:18.470|03B|     </getitemestimate:Response>
14:04:18.470|03B|   </getitemestimate:GetItemEstimate>
14:04:18.470|03B|   
14:04:18.470|03B|   Sync
14:04:18.470|03B|   <?xml version="1.0" encoding="utf-8"?>
14:04:18.470|03B|   <Sync xmlns="AirSync:">
14:04:18.470|03B|     <Collections>
14:04:18.470|03B|       <Collection>
14:04:18.470|03B|         <SyncKey>2521180</SyncKey>
14:04:18.470|03B|         <CollectionId>62</CollectionId>
14:04:18.471|03B|         <GetChanges />
14:04:18.471|03B|         <WindowSize>50</WindowSize>
14:04:18.471|03B|         <Options>
14:04:18.471|03B|           <BodyPreference xmlns="AirSyncBase:">
14:04:18.471|03B|             <Type>2</Type>
14:04:18.471|03B|           </BodyPreference>
14:04:18.471|03B|           <Conflict>0</Conflict>
14:04:18.471|03B|         </Options>
14:04:18.471|03B|       </Collection>
14:04:18.471|03B|     </Collections>
14:04:18.471|03B|   </Sync>

This is all expected. 10 minutes later it does a sync again:

14:04:18.471|03B|   FolderSync
14:14:17.358|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:17.359|03B|   <FolderSync xmlns="FolderHierarchy:">
14:14:17.359|03B|     <SyncKey>0</SyncKey>
14:14:17.359|03B|   </FolderSync>
[...]

The calendar sync works fine:

14:14:18.199|03B|   GetItemEstimate
14:14:18.201|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:18.202|03B|   <GetItemEstimate xmlns="GetItemEstimate:">
14:14:18.202|03B|     <Collections>
14:14:18.202|03B|       <Collection>
14:14:18.202|03B|         <SyncKey xmlns="AirSync:">1305192833</SyncKey>
14:14:18.202|03B|         <CollectionId>5</CollectionId>
14:14:18.202|03B|       </Collection>
14:14:18.202|03B|     </Collections>
14:14:18.202|03B|   </GetItemEstimate>
14:14:18.203|03B|   
14:14:18.203|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:18.316|03B|   <getitemestimate:GetItemEstimate xmlns:getitemestimate="GetItemEstimate:">
14:14:18.316|03B|     <getitemestimate:Response>
14:14:18.316|03B|       <getitemestimate:Status>1</getitemestimate:Status>
14:14:18.316|03B|       <getitemestimate:Collection>
14:14:18.316|03B|         <getitemestimate:CollectionId>5</getitemestimate:CollectionId>
14:14:18.316|03B|         <getitemestimate:Estimate>0</getitemestimate:Estimate>
14:14:18.316|03B|       </getitemestimate:Collection>
14:14:18.316|03B|     </getitemestimate:Response>
14:14:18.316|03B|   </getitemestimate:GetItemEstimate>
14:14:18.317|03B|   
14:14:18.317|03B|   Sync
14:14:18.317|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:18.317|03B|   <Sync xmlns="AirSync:">
14:14:18.317|03B|     <Collections>
14:14:18.317|03B|       <Collection>
14:14:18.317|03B|         <SyncKey>1305192833</SyncKey>
14:14:18.317|03B|         <CollectionId>5</CollectionId>
14:14:18.317|03B|         <GetChanges />
14:14:18.317|03B|         <WindowSize>50</WindowSize>
14:14:18.317|03B|         <Options>
14:14:18.317|03B|           <BodyPreference xmlns="AirSyncBase:">
14:14:18.317|03B|             <Type>2</Type>
14:14:18.317|03B|           </BodyPreference>
14:14:18.317|03B|           <Conflict>0</Conflict>
14:14:18.317|03B|         </Options>
14:14:18.317|03B|       </Collection>
14:14:18.317|03B|     </Collections>
14:14:18.317|03B|   </Sync>

But for some reason, Contacts no longer works. It gets a Status of 4 (invalid SyncKey) for GetItemEstimate and a Status of 3 (invalid SyncKey) for Sync:

14:14:18.317|03B|   GetItemEstimate
14:14:18.484|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:18.484|03B|   <GetItemEstimate xmlns="GetItemEstimate:">
14:14:18.484|03B|     <Collections>
14:14:18.484|03B|       <Collection>
14:14:18.484|03B|         <SyncKey xmlns="AirSync:">395211212</SyncKey>
14:14:18.484|03B|         <CollectionId>10</CollectionId>
14:14:18.484|03B|       </Collection>
14:14:18.485|03B|     </Collections>
14:14:18.485|03B|   </GetItemEstimate>
14:14:18.485|03B|   
14:14:18.485|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:18.607|03B|   <getitemestimate:GetItemEstimate xmlns:getitemestimate="GetItemEstimate:">
14:14:18.607|03B|     <getitemestimate:Response>
14:14:18.607|03B|       <getitemestimate:Status>4</getitemestimate:Status>
14:14:18.607|03B|       <getitemestimate:Collection>
14:14:18.607|03B|         <getitemestimate:CollectionId>10</getitemestimate:CollectionId>
14:14:18.607|03B|       </getitemestimate:Collection>
14:14:18.607|03B|     </getitemestimate:Response>
14:14:18.607|03B|   </getitemestimate:GetItemEstimate>
14:14:18.607|03B|   
14:14:18.607|03B|   Sync
14:14:18.608|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:18.608|03B|   <Sync xmlns="AirSync:">
14:14:18.608|03B|     <Collections>
14:14:18.608|03B|       <Collection>
14:14:18.608|03B|         <SyncKey>395211212</SyncKey>
14:14:18.608|03B|         <CollectionId>10</CollectionId>
14:14:18.608|03B|         <GetChanges />
14:14:18.608|03B|         <WindowSize>50</WindowSize>
14:14:18.608|03B|         <Options>
14:14:18.608|03B|           <BodyPreference xmlns="AirSyncBase:">
14:14:18.608|03B|             <Type>2</Type>
14:14:18.608|03B|           </BodyPreference>
14:14:18.608|03B|           <Conflict>0</Conflict>
14:14:18.608|03B|         </Options>
14:14:18.608|03B|       </Collection>
14:14:18.608|03B|     </Collections>
14:14:18.608|03B|   </Sync>
14:14:18.608|03B|   
14:14:18.608|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:18.775|03B|   <airsync:Sync xmlns:airsync="AirSync:">
14:14:18.775|03B|     <airsync:Collections>
14:14:18.775|03B|       <airsync:Collection>
14:14:18.775|03B|         <airsync:SyncKey>395211212</airsync:SyncKey>
14:14:18.775|03B|         <airsync:CollectionId>10</airsync:CollectionId>
14:14:18.775|03B|         <airsync:Status>3</airsync:Status>
14:14:18.775|03B|       </airsync:Collection>
14:14:18.775|03B|     </airsync:Collections>
14:14:18.775|03B|   </airsync:Sync>

It immediately tries to resync all Contacts again, with a SyncKey of 0 to restart the sync:

14:14:18.775|03B|   Sync
14:14:18.775|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:18.775|03B|   <Sync xmlns="AirSync:">
14:14:18.775|03B|     <Collections>
14:14:18.775|03B|       <Collection>
14:14:18.776|03B|         <SyncKey>0</SyncKey>
14:14:18.776|03B|         <CollectionId>10</CollectionId>
14:14:18.776|03B|         <Options>
14:14:18.776|03B|           <BodyPreference xmlns="AirSyncBase:">
14:14:18.776|03B|             <Type>2</Type>
14:14:18.776|03B|           </BodyPreference>
14:14:18.776|03B|           <Conflict>0</Conflict>
14:14:18.776|03B|         </Options>
14:14:18.776|03B|       </Collection>
14:14:18.776|03B|     </Collections>
14:14:18.776|03B|   </Sync>
14:14:18.776|03B|   
14:14:18.776|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:19.005|03B|   <airsync:Sync xmlns:airsync="AirSync:">
14:14:19.005|03B|     <airsync:Collections>
14:14:19.005|03B|       <airsync:Collection>
14:14:19.005|03B|         <airsync:SyncKey>1634521428</airsync:SyncKey>
14:14:19.005|03B|         <airsync:CollectionId>10</airsync:CollectionId>
14:14:19.005|03B|         <airsync:Status>1</airsync:Status>
14:14:19.005|03B|       </airsync:Collection>
14:14:19.005|03B|     </airsync:Collections>
14:14:19.005|03B|   </airsync:Sync>
14:14:19.005|03B|   
14:14:19.005|03B|   Sync
14:14:19.008|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:19.008|03B|   <Sync xmlns="AirSync:">
14:14:19.008|03B|     <Collections>
14:14:19.008|03B|       <Collection>
14:14:19.008|03B|         <SyncKey>1634521428</SyncKey>
14:14:19.008|03B|         <CollectionId>10</CollectionId>
14:14:19.008|03B|         <GetChanges />
14:14:19.008|03B|         <WindowSize>50</WindowSize>
14:14:19.008|03B|         <Options>
14:14:19.008|03B|           <BodyPreference xmlns="AirSyncBase:">
14:14:19.008|03B|             <Type>2</Type>
14:14:19.008|03B|           </BodyPreference>
14:14:19.008|03B|           <Conflict>0</Conflict>
14:14:19.008|03B|         </Options>
14:14:19.008|03B|       </Collection>
14:14:19.008|03B|     </Collections>
14:14:19.008|03B|   </Sync>
14:14:19.008|03B|   
14:14:19.008|03B|   <?xml version="1.0" encoding="utf-8"?>
14:14:19.751|03B|   <airsync:Sync xmlns:airsync="AirSync:">
14:14:19.751|03B|     <airsync:Collections>
14:14:19.751|03B|       <airsync:Collection>
14:14:19.751|03B|         <airsync:SyncKey>1071299936</airsync:SyncKey>
14:14:19.751|03B|         <airsync:CollectionId>10</airsync:CollectionId>
14:14:19.751|03B|         <airsync:Status>1</airsync:Status>
14:14:19.751|03B|         <airsync:MoreAvailable />
[...]

It continues syncing and completes it, then syncs Tasks too. 10 minutes later, it does the same but this time the calendar sync fails in the same way:

14:24:18.140|03B|   GetItemEstimate
14:24:18.141|03B|   <?xml version="1.0" encoding="utf-8"?>
14:24:18.141|03B|   <GetItemEstimate xmlns="GetItemEstimate:">
14:24:18.141|03B|     <Collections>
14:24:18.141|03B|       <Collection>
14:24:18.141|03B|         <SyncKey xmlns="AirSync:">1305192833</SyncKey>
14:24:18.142|03B|         <CollectionId>5</CollectionId>
14:24:18.142|03B|       </Collection>
14:24:18.142|03B|     </Collections>
14:24:18.142|03B|   </GetItemEstimate>
14:24:18.142|03B|   
14:24:18.142|03B|   <?xml version="1.0" encoding="utf-8"?>
14:24:18.288|03B|   <getitemestimate:GetItemEstimate xmlns:getitemestimate="GetItemEstimate:">
14:24:18.288|03B|     <getitemestimate:Response>
14:24:18.288|03B|       <getitemestimate:Status>4</getitemestimate:Status>
14:24:18.288|03B|       <getitemestimate:Collection>
14:24:18.288|03B|         <getitemestimate:CollectionId>5</getitemestimate:CollectionId>
14:24:18.288|03B|       </getitemestimate:Collection>
14:24:18.288|03B|     </getitemestimate:Response>
14:24:18.288|03B|   </getitemestimate:GetItemEstimate>
14:24:18.288|03B|   
14:24:18.289|03B|   Sync
14:24:18.289|03B|   <?xml version="1.0" encoding="utf-8"?>
14:24:18.289|03B|   <Sync xmlns="AirSync:">
14:24:18.289|03B|     <Collections>
14:24:18.289|03B|       <Collection>
14:24:18.289|03B|         <SyncKey>1305192833</SyncKey>
14:24:18.289|03B|         <CollectionId>5</CollectionId>
14:24:18.289|03B|         <GetChanges />
14:24:18.289|03B|         <WindowSize>50</WindowSize>
14:24:18.289|03B|         <Options>
14:24:18.289|03B|           <BodyPreference xmlns="AirSyncBase:">
14:24:18.289|03B|             <Type>2</Type>
14:24:18.289|03B|           </BodyPreference>
14:24:18.289|03B|           <Conflict>0</Conflict>
14:24:18.289|03B|         </Options>
14:24:18.289|03B|       </Collection>
14:24:18.289|03B|     </Collections>
14:24:18.289|03B|   </Sync>
14:24:18.289|03B|   
14:24:18.289|03B|   <?xml version="1.0" encoding="utf-8"?>
14:24:18.743|03B|   <airsync:Sync xmlns:airsync="AirSync:">
14:24:18.743|03B|     <airsync:Collections>
14:24:18.743|03B|       <airsync:Collection>
14:24:18.743|03B|         <airsync:SyncKey>1305192833</airsync:SyncKey>
14:24:18.743|03B|         <airsync:CollectionId>5</airsync:CollectionId>
14:24:18.743|03B|         <airsync:Status>3</airsync:Status>
14:24:18.743|03B|       </airsync:Collection>
14:24:18.743|03B|     </airsync:Collections>
14:24:18.743|03B|   </airsync:Sync>

It retries the Sync but this time it gets a Status 132, which is undocumented. My guess is it might have hit some kind of rate limit.

14:24:18.743|03B|   Sync
14:24:18.743|03B|   <?xml version="1.0" encoding="utf-8"?>
14:24:18.743|03B|   <Sync xmlns="AirSync:">
14:24:18.743|03B|     <Collections>
14:24:18.743|03B|       <Collection>
14:24:18.743|03B|         <SyncKey>0</SyncKey>
14:24:18.743|03B|         <CollectionId>5</CollectionId>
14:24:18.743|03B|         <Options>
14:24:18.743|03B|           <BodyPreference xmlns="AirSyncBase:">
14:24:18.743|03B|             <Type>2</Type>
14:24:18.743|03B|           </BodyPreference>
14:24:18.743|03B|           <Conflict>0</Conflict>
14:24:18.743|03B|         </Options>
14:24:18.743|03B|       </Collection>
14:24:18.743|03B|     </Collections>
14:24:18.744|03B|   </Sync>
14:24:18.744|03B|   
14:24:18.744|03B|   <?xml version="1.0" encoding="utf-8"?>
14:24:22.173|03B|   <airsync:Sync xmlns:airsync="AirSync:">
14:24:22.173|03B|     <airsync:Status>132</airsync:Status>
14:24:22.173|03B|   </airsync:Sync>

It retries several times, getting mostly the 132’s and sometimes no response, but ultimately throws the exception:

14:24:35.880|03B|   Web exception: Exception of type 'MailClient.Protocols.Common.HttpResponseException' was thrown.
14:24:36.052|03B|   
14:24:36.052|03B|   Exception: MailClient.Storage.Application.OperationException: Synchronizing folder 'Personal/Calendar' failed due to the following error: Exception of type 'MailClient.Protocols.Common.HttpResponseException' was thrown.
14:24:36.052|03B|    ---> MailClient.Protocols.Common.HttpResponseException: Exception of type 'MailClient.Protocols.Common.HttpResponseException' was thrown.
14:24:36.053|03B|      at MailClient.Protocols.Common.WebClientExtensions.EnsureOK(HttpResponseMessage response)
14:24:36.053|03B|      at MailClient.Protocols.AirSync.Protocol.ActiveSyncCommandRequest.GetResponse(CancellationToken cancellationToken)
14:24:36.053|03B|      at MailClient.Protocols.AirSync.Protocol.LoggingCommandRequest.GetResponse(CancellationToken cancellationToken)
14:24:36.053|03B|      at MailClient.Protocols.AirSync.AirSyncItemSynchronizer`2.GetSyncKey(Folder folder, IItemContext context, CancellationToken cancellationToken)
14:24:36.053|03B|      at MailClient.Protocols.AirSync.AirSyncItemSynchronizer`2.Synchronize(IItemSynchronizeContext`1 synchronizeContext, Folder folder, CancellationToken cancellationToken)
14:24:36.053|03B|      at MailClient.Protocols.Common.ItemSynchronizer`2.<>c__DisplayClass37_2.<EnqueueSynchronize>b__6(WorkerStatus status, CancellationToken ct)
14:24:36.053|03B|      at MailClient.Protocols.AirSync.AirSyncGenericCommand.Execute(WorkerStatus status)
14:24:36.053|03B|      --- End of inner exception stack trace ---
14:24:36.053|03B|      at MailClient.Protocols.AirSync.AirSyncGenericCommand.Execute(WorkerStatus status)
14:24:36.053|03B|      at MailClient.Commands.Command.Process(WorkerStatus status)
14:24:36.053|03B|   AccountBase.GoOfflineAsync : State changed to OFFLINE due BrokenConnection
14:24:36.053|01B|   AccountBase.ChangeOnlineState : State changed to OFFLINE due BrokenConnection

There is something flaky about the state of the SyncKeys, but it also seems that the implementation of FolderSync, GetItemEstimate, and Sync is a little strange.

One thing I noted from the example second response in the docs is that the GetChanges element should be <GetChanges>1</GetChanges>, and not <GetChanges />, which might imply a null value.

Another thing I noted was the protocol for handling “nothing to sync” is quite different from how eM Client is handling it. It should be sending an empty Sync request as defined here. While the spec doesn’t restrict sending an XML body, it certainly does not mention using GetItemEstimate while there is an empty response.

Lastly, I wonder whether this has anything to do with multiple devices with eM Client running. SyncKeys shouldn’t conflict as each device will maintain its key, but who knows.

mre · March 23, 2026, 4:23pm

I’ve been having this problem for years. Emclient denies it but the calendar stopped working for me long ago. Doesn’t work in the ios app either. I switched to Betterbird years ago.

eMDash · March 25, 2026, 7:45pm

I realise there are differences between the services Microsoft offers, as well as their mechanisms for delivery. Just for comparison, I can add that I use Exchange services (Office 365 package) and the calendar seems to work fine for me for most needs. The only thing I’ve not got to the bottom of is the use of shared/delegated calendars after following the guidance (but I won’t derail the thread).

MarkyBoy · April 12, 2026, 11:00pm

I have the same calendar airsync problem and also have had to revert to classic Outlook. New Outlook is a mess and asking which is being used is an irrelevant question because they both do sync calendars correctly. The problem is clearly not a Microsoft outage.

The problem is with emClient (Windows and iOS version at least).

I appreciate that Microsoft could be constantly messing with their protocols because they are so bad at getting things right first time - but…. If emClient offers syncing with Outlook accounts then it should be fixed!

Disappointing because it is such a good product when it works.

cyberzork · April 12, 2026, 11:05pm

If you have V10 and originally installed eM Client from the website, there is now a newer updated V10.4.5326 now available for PC & Mac on the release history pages you can try and see if that helps. It’s a service update and bug fixes.

Before updating, make a manual backup first via “Menu / Backup” incase you need to restore for any reason. Also close eM Client before updating.