I set up an IMAP account, yesterday he downloaded all the mail on the server, this morning he can’t synchronize the folders.
If you go to Menu > Tools > Operations, look for any IMAP errors in the Log tab.
So there are no errors.
What is it that is the problem?
It may still be busy with the initial sync. You could change the settings and see if there is any difference. Go to Menu > Tools > Accounts and click on the IMAP tab. Change the port to 993 and the security policy to Use SSL/TLS on special port (legacy). Restart eM Client.
Or, if you have a Pro License, you can open a support ticket with eM Client. It may have something to do with your company server, and they will be able to help you further.