I started today having exactly same problem. Times 5, that is: I have five Yahoo accounts, which worked fine earlier with eM Client. Not anymore.
This error is a Yahoo mail server type error and you need to contact their technical support.
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Thank you so much, cyberzork. Appreciated. 
Once again we see that the best thing to do is NEVER use Yahoo email. For example Outlook and Gmail always work fine with eM Client and “standalone”. Yahoo Email is a relic that should be removed from the earth for good.
Thanks.
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Unfortunately it’s well known that EmClient encounters problems with the integration of yahoo account 
This is not an eM Client issue, but a problem with the Yahoo! server. Please contact your email provider as their server is broken.
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Is it possible to simply not to display or hide the error ? An option somewhere in Em Client ?
I mean, despite the error log everything works fine and when I click to ignore, next time this reappears again… it’s a never ending story !
Maybe the easy way it’s just not to display the error 
17:34:31 solitaire2511@yahoo.fr IMAP Synchronisation de la liste des dossiers
17:34:40 solitaire2511@yahoo.fr [IMAP] MailClient.Protocols.Imap.ImapException: Failed to LIST server folders ( LIST Invalid credentials)
at MailClient.Protocols.Imap.ImapFolderSynchronizer.ListFoldersAsync(IListFoldersProgress progress, Folder homeFolder, CancellationToken cancellationToken)
at MailClient.Protocols.Imap.ImapFolderSynchronizer.ListFoldersAsync(IListFoldersProgress progress, Folder homeFolder, CancellationToken cancellationToken)
at MailClient.Protocols.Imap.ImapFolderSynchronizer.GetFolderList(IListFoldersProgress progress, Folder homeFolder, CancellationToken cancellationToken)
at MailClient.Protocols.Common.FolderSynchronizer.<>c__DisplayClass14_1.b__0(WorkerStatus status, CancellationToken cancellationToken)
at MailClient.Protocols.Imap.ImapActionCommand.<>c__DisplayClass13_0.<.ctor>b__0(WorkerStatus ws, CancellationToken ct)
at MailClient.Protocols.Imap.ImapActionCommand.ExecuteInternalSync(WorkerStatus status)
at MailClient.Protocols.Imap.ImapActionCommand.Execute(WorkerStatus status)
at MailClient.Commands.Command.Process(WorkerStatus status)
— End of stack trace from previous location —
at MailClient.Storage.Synchronization.FolderSynchronizerExtensions.SynchronizeSubfoldersAsync(IFolderSynchronizer synchronizer, Folder folder, SynchronizationPriority priority, CancellationToken cancellationToken)
at MailClient.Protocols.Imap.ImapAccount.SynchronizeIntAsync(Boolean forced)
at MailClient.Protocols.Common.SynchronizableMailAccount.<>c__DisplayClass6_0.<b__0>d.MoveNext()
reason_code = 0
EnqueuedStackTrace =
at MailClient.Protocols.Common.FolderSynchronizer.EnqueueGetFolderList(Folder homeFolder, Action1 completed) at MailClient.Protocols.Common.FolderSynchronizer.MailClient.Storage.Synchronization.IFolderSynchronizer.SynchronizeSubfolders(Folder folder, SynchronizationPriority priority, Action1 completed)
at MailClient.Storage.Synchronization.FolderSynchronizerExtensions.<>c__DisplayClass0_0.b__1()
at System.Threading.Tasks.Task.InnerInvoke()
at System.Threading.Tasks.Task.<>c.<.cctor>b__277_0(Object obj)
at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
at System.Threading.Tasks.Task.ExecuteEntryUnsafe(Thread threadPoolThread)
at System.Threading.Tasks.Task.ExecuteFromThreadPool(Thread threadPoolThread)
at System.Threading.ThreadPoolWorkQueue.Dispatch()
at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()
GUIStatus_Exception_Reported = True
Thanks for the fast reply 
Thank you. I experience the same problem with error messages popping up but I am not seeing any fatal errors. I was hoping there was a way to stop the popups
[IMAP] NO [AUTHENTICATIONFAILED] APPEND Invalid credentials
[IMAP] NO [AUTHENTICATIONFAILED] APPEND Invalid credentials
[IMAP] Cannot open folder “/Inbox/Panama/Miranda Contreras/Larissa de Gracia”. (NO [AUTHENTICATIONFAILED] SELECT Invalid credentials)
[IMAP] Cannot open folder “/Inbox”. (NO [AUTHENTICATIONFAILED] SELECT Invalid credentials)
[IMAP] Failed to LIST server folders ( LIST Invalid credentials)
[IMAP] Cannot open folder “/Inbox/Frontline Law/Julie Schmidt”. (NO [AUTHENTICATIONFAILED] SELECT Invalid credentials)
I get these all the time. Has been happening for several months.
Thanks.
[IMAP] NO [AUTHENTICATIONFAILED] APPEND Invalid credentials
[IMAP] Failed to LIST server folders ( LIST Invalid credentials
These are Yahoo server errors “I too now have been getting these same errors alot of the time” since mine initially worked ok using the OAuth setup. It’s a Yahoo server problem and nothing to do with eM Client. Happens with any version of eM Client.
I’ve emailed Yahoo a few times about this but never get a reply back from their technical support. I reckon the Yahoo servers need updating or patching but they are not in a hurry to fix it.
As @ProComposer says above “Outlook and Gmail always work fine” with eM Client and “standalone”. Yahoo Email is a relic that should be removed from the earth for good
This happens to me too.
[IMAP] Cannot open folder “/Inbox”. (NO [AUTHENTICATIONFAILED] SELECT Invalid credentials)
Oddly, it continues to sync messages just fine, though not always fast sometimes.
I found an article at Yahoo:
It says “If you receive an error message like “Server password has changed” or “Authentication failed,” your app might be outdated. The app likely has outdated security protocols and are disabled by default.”
So according to Yahoo eM is using outdated protocols, rather than Yahoo servers having outdated software or something.
If I follow further, go here:
It says: “If you try to use Yahoo Mail in a third-party email client using less secure sign-in technology, you might get a “Server password has changed” or an “Authentication failed” error message. This is because those email apps use outdated security protocols and we’ve disabled access to them by default now.”
Well, we do get “Authentication failed” messages, so again eM must be using outdated security protocols.
It also says if people have an account key or use two-step, this can be an issue and might be the cause for some people. For me personally, I don’t have those turned on, it’s just my user/pass is all.
Before eM makes the random claim Yahoo’s servers are bad/old/outdated as if they had like one server somewhere on GoDaddy they forgot to update. They have a massive infrastructure like anybody else. They have 225 million users sending/receiving 26 billion messages a day. It’s not likely Bob the IT guy forgot to run apt-upgrade.
I don’t know who is ultimately at fault. Yahoo can connect with the oauth stuff, or an app password, but it doesn’t seem likely it’s their server being out of date. As others have mentioned, there are plenty of other email apps that connect to Yahoo fine while eM doesn’t.
I’m not here to blame because I have not clue which is at fault, but I’m going to guess 225 million people using Yahoo every day from all sorts of clients are doing just fine.
I would at least suggest eM try their darndest to figure out if there is something up with protocols or SSL or encryption keys or storage of data or character encoding or any possible way to make the connection can be a little more robust.
I know it’s easy to say “the server sent an error, so it’s there fault.” Ya, the error is that authentication failed, so why is the authentication failing that eM is trying to send/use?
If I try to log in to Gmail and type the wrong password and it says “authentication failed” I don’t just say “well Gmail’s servers are sending errors, must be their fault”.
Yahoo is only about 7 years older than Gmail, and Gmail itself is practically “ancient” from 2004. So just saying a service is old doesn’t mean anything. They clearly use 2FA and oauth like everybody else.
All I ask is spend a bit more time being absolutely positive, because other email clients continue to work fine while eM doesn’t. Even if it’s just a server error, it’s not like some code bug or accident, there is a reason for the error, something wrong in the communication standards or something. Hopefully eM can work around it, even use “older” protocols or techniques if that’s what it takes.
Anyway, since the email continues to sync, the error hardly makes sense, so I disabled the error popups out of convenience.
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This is a server error. You will need to contact Yahoo! for assistance as their server is broken.
But even though it is a server problem, one thing you can try is installing version 9.1, where we have done some work to reduce the chance of this happening through some workarounds.
You can download the latest version in the Release History
Yes the server sent an error that the credentials were invalid. So why is eM sending invalid credentials?
I can’t accept that the “server is broke”, Yahoo is processing billions of emails a day. eM is still sending/receiving just fine regardless. They probably run entire server farms for this, not one server in some random place. I’d say about 5 to 10 times a day this error is triggered, it seems quite random and doesn’t have any real effect on processing messages. I can’t think of any repetitive task that could be triggering it. Heck, it usually happens when I’m not even using it, like when it’s just sitting in the Windows system tray in the background.
I have it set to check for new mail every 10 minutes. I doubt that is hitting some kind of rate limit.
Hopefully some kind of fix is discovered eventually.
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eM Client isn’t sending invalid credentials. It is the server that is rejecting valid credentials, so broken. That is because it first accepts the credentials, and then immediately rejects them again.
Just because a server process x number of messages doesn’t mean it can’t be broken. You have the error message the server sent you, so please ask Yahoo! to fix it. We have a possible workaround, so you may not see the error again, but the server is still broken.
Since updating EmClient to 9.1.2053.0 I. haven´t received any error messages
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I have a hard time accepting that answer Gary. I mean you could be right. But just blantantly suggest a gazillion dollar company that processes billions of emails is operating with “broken” servers doesn’t make a lick of sense.
Are you saying everyone on eM with a Yahoo account is reporting this, it happens to everyone? That would make sense if their servers are broken. But how come other email clients don’t have the issue? How come a lot of eM users aren’t reporting this? It seems like it must work for a majority of people then?
It would seem that Yahoo’s servers only seem to be broken for eM users, and then only some of those users. It doesn’t make sense.
And just what am I supposed to report to Yahoo about their servers? Do you know which server it is? Is there an IP or domain name for the specific server that isn’t working right? I assume they must use some kind of load balancing or DNS red robin technique so maybe it’s only particular servers that have hiccups? What data can I give them to report except “there is an email client I use and they say your servers are broken”.
I can already predict their answer: “Nobody else reporting broken servers and email is functioning fine, must be something wrong with your email client, our authentication servers don’t show any issues.”
I don’t want to get stuck in he-said-she-said.
And if the server is broken, I don’t see how you just “made a workaround” in the latest version. So you fixed eM to properly work with Yahoo?
None of this makes sense. I think your own programming team might be better situated to report Yahoo’s broken servers. After all you are trying to program according to their APIs and authentication protocols I assume, so if all that isn’t working, you should report it to them.
I don’t know what I’m supposed to do about it, that’s all. And Yahoo isn’t going to agree anything is wrong with their server unless I have some kind of technical proof. At least a log or something.
I’ll help if I can.
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Broken may be a poor choice of word, so maybe this will explain it.
After about an hour (Yahoo! might have increased it because of the backlash) their server rejects your credentials as invalid and any operation on an active imap connection will result in an error. So for the past hour the server has accepted them, but suddenly it refuses. You can see that in your error where the server says NO - invalid credentials. The correct solution would be for server to close any active connection if it no longer considers them valid, but this would cost computing power, so maybe that had something to do with their choice.
We did however, provide a workaround of sorts for the server’s behavior. Before we implemented this, the default method was for eM Client to retry any pending operation on a different active connection. Which unsurprisingly would also fail. So now, after 2 failed attempts the account goes offline. The error is still there but eM Client will be offline for a while.
Does it have something to do with how frequently we have it check for new messages and such? Like I have it set to 10 minutes, but if Yahoo is timing it out or something, maybe I actually need to check more frequently to keep the connections alive? Or are they killing the connections after an hour no matter how active it is?
Maybe eM should be completely closing the connection whenever there isn’t an active task happening, like only sync up for an operation or check messages and then close it again. Heck I don’t know. I know Yahoo is a pain no matter what, but it’s hard to get rid of ancient accounts.
I have gotten these errors myself when using eM Client and Yahoo. Since the errors were sporadic and didn’t happen all the time, I just assumed it was issues with Yahoo. Now in days it seems like Yahoo doesn’t care for the up keep of their email services. That is why I would never consider using Yahoo as my primary email hosting service.
What I can say is since installing the latest version (9.1) I have not gotten those errors. I absolutely understand how a work around could fix issues caused by the server end and just thankful they even took the time to work around Yahoo’s server issues.
I think at this point most of the major email hosting providers are trying to push users into using their own WebMail, and don’t want to support standards like IMAP and POP very well. They then force clients into using their proprietary APIs which can be ugly especially when it is in a consistent change. This means you then have to support things that are not standard except to that service itself. So I can understand how all this can be difficult and a pain for email client developers when everyone is doing their own thing in a non-standardized way.
I just wanted to make it known that there are eM Client users having these Yahoo server issues, and agree that it is Yahoo’s fault, and might not report it because Yahoo is not our primary email service and just have ignored these sporadic server errors as they pop-up to cope with their issues. Of course like I said ignoring the errors is no longer an issue since the work around in 9.1 seems to have resolved getting these errors.
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Yeap me too, I’ve removed the Sky/Yahoo account from eMclient and revoked the Oauth in the yahoo settings page (recent activity) and the re entered the account to eMclient and authenticated with Yahoo.
Seemed ok for a couple of days. The just now it’s tried a sync and failed with the unable to get LIST of folders!
I see the 9.1 update is suppose to have a work around. but if I check for updates in eMclient (I’m on 9.0) it says none available.
Any other suggestions/help?