Thats very unusual its still happening. Could be eg: you might have an older eM Client version and might need updating, or you might have some external running resident is possibly corrupting your eM Client database.
So first if you originally installed eM Client from the eM Client webpage, then you can check if you are running the latest version for whatever you currently use via the release history page..
Note:- If you do update to a later version, make a backup first via “Menu / Backup” incase you need to restore for any reason. Also close eM Client before updating.
Next if you already have the latest eM Client version, then i would next suggest to “try a new clean database” if you haven’t already. To do that first “close eM Client” and then go to the following hidden database folder and rename it to eg: eM Client-Current .
(Hidden database folder Windows)
C:\Users%username%\AppData\Roaming\eM Client
(Hidden database folder Mac)
/Users/User Name/Library/Application Support/eM Client
Then once renamed “reopen eM Client” and setup as new and see if that fixes the problem. If you then need any messages etc from your previous current database, see my post how to do a Partial Restore.
Apart from that, “if you have been making regular eM Client backups” either automatically via “Menu / Settings / General / Backup” or manually via “Menu / Backup” then you could alternatively restore a recent backup when you know it was working ok via “Menu / File / Restore” and any new emails since the backup will update when eM Client reconnects. Then if that works, make another new backup.