When using an account that is running on an Exchange server provided by Microsoft, you can encounter this error:

Known Microsoft Exchange server error "User is authenticated but not connected" has been encountered on IMAP connection. This issue is temporary and should be automatically resolved by the server. If not, please contact your mail provider.

This error means that the authentication process for your account was succesfull, but for some reason the mail server is not giving eM Client access to the account data, so it cannot synchronize your messages into the app. Unfortunately, this is not something we can resolve from our side, it needs to be resolved from the side of the mail server that actually contains all your data. Reach out to your admin or provider of the email account so they can fix the issue.

If this is happening with an email account that is set up using an Alias and not the Primary email address, try setting up the account anew using the primary email address to see if that resolves the problem.