Hello, I started using eM Client for the first time at the start of the month (Sep '21) using the Free version. I backup eMC and today checked the files in both folders and notice a discrepancy between a file in the local folders: mail_data.dat - the file size is 2.84GB, but the backup file size is only 10,408 KB? There are others but this is the largest discrepancy. I wanted to check in case there’s a problem, and if so, what can I do about it. Thanks.
Backup file is compressed (ZIP)
Uncompressed file sizes. But I’d love to have software that could compress a 3GB file down to 10MB. 
Well perhaps someone will come up with a better answer…
I thought eMC reads and responds to posts? I searched through existing posts and couldn’t find anything under any existing topics and threads so thought eMC would be the best people to have the answer. Thanks for trying though.
You need to purchase that capability.
The forum is free and I’ve seen them reply on here.
A discrepancy between a file in the local folders: mail_data.dat - the file size is 2.84GB
I suspect the Mail_data.dat file you are looking for is in (another folder) with eM Client Backup.zip.
There is multiple folders with “Mail_data.dat” in the backup.zip (apart from the local folder) in there, where some Mail_data.dat files are only very small kb in size, where others are very large in size.
So look in (all the folders within the backup) including the long folders with combinations of letters and numbers. Example below of where alot of differant Mail_data.dat files are located in the backup.zip
Thanks for your reply. Unfortunately I’d already done that. Thanks anyway.
assuming you keep multiple backups, check the dates and are they essentially slowly getting bigger, have correct dates, and are all around 10 meg?
Does 2.8 GB seem potentially reasonable for your email? How big was your local folder in your previous email program.
FYI - I checked my mail_data.dat local folder and it was essentially the same size as the zipped backup because of all the extra info the zipped file has, so, no, compression isn’t the reason.
I used MS Outlook previously which included calendar, tasks, contacts, etc., and I’ve moved quite a number of older emails into the ‘Archive’ folder. Even if we work on the assumption that the size of the file in the backup folder is actually correct at 10 MB we are still left with a file in the ‘Local Folders’ of 2.8 GB. One of these isn’t correct.
My main issue was if there were emails which weren’t being backed up, and therefore would be lost if I needed to restore, but if all emails are backed up and the issue is one of a file size being incorrect at 2.8 GB then that’s a different issue.
If it’s just a glitch with no repercussions, I can live with it, but as a new eMClient user I’m obviously overly cautious to make sure everything’s in working order. Thanks for the reply, appreciate it! 
No, everything is there. A backup is a complete snapshot of eM Client as it exists at that moment. It includes all data, settings and preferences.
The size of working files within a database folder is not an accurate representation of the actual data as they contain temporary data as well. The backup will rebuild and verify the data, and save it as a compressed clean copy.
You can verify that yourself.
Disconnect from the Internet, and make a backup using Menu > Backup, then close eM Client and rename the database directory. By default that is C:\Users\yourusername\AppData\Roaming\eM Client\ You may have to enable hidden items in Explorer to get there.
Restart eM Client.
You will be in a blank new database. Cancel any setup windows and go to Menu > File > Restore. Select the backup you just made.
When the restore is complete check your data.
When you are satisfied everything is there, you can close eM Client, delete the new C:\Users\yourusername\AppData\Roaming\eM Client\ folder and rename the old one back to \eM Client\
Reconnect to the Internet, and restart eM Client.
You will now be back in your original database.
@Gary and @MatthewLloyd it seems to me that this is still odd. The only explanation I can think of is the archive is not part of the backup or lots of stuff, probably with big attachments was deleted. I don’t know, but assume eMclient is using sqLite which, unless a vacuum command is given retains all the old deleted items and will re-use the space as new emails come in. I assume that if we had a 2.8 gb database and deleted 2 gb we’d still have a 2.8 gb database with lots of space for further growth.
Is this possible? and do we have an ability to compress the database and remove the excessive emptyish records?
Thanks very much for this, Gary. I will check it out over the coming weekend.
Also, thanks @steveshank. Forgive my ignorance but would compressing the database improve performance? Or is it more about housekeeping?
Thanks for the replies, appreciate it.
No, everything is there. Nothing is deleted.
No, everything is there. Nothing is deleted.
Gary, I wasn’t clear. If the end user deleted lots of big files, like video attachments and then he backed up, then the backup might be much smaller because those files were no longer there. While the database would retain the space for future use.
Anyway, that was the way I was looking at finding an explanation for the huge size difference.
@MatthewLloyd - I honestly don’t know if it would improve performance. I doubt it. But such a huge amount of wasted space makes me unhappy.