My new mailserver is up and running. I like slow migrations and just migrated one of my last domains.
The total number of imap-accounts in mail.app reached 4. For every account I selected “Automatically synchronize changed mailboxes”.
And at every start of mail.app, the application comes to a complete deadlock.<div align=center>

Courier-imaps settings in mail.app </div> I was afraid it showed up to be an incompatibility with courier-imap, mail.app and synchronizing subfolders.

It took me a full day googling, hoping and lucky guesses.
The solution:

Max. of three accounts for a full sync. of all subfolders.
So you can add more accounts, but just take care that you not select “Automatically synchronize changed mailboxes”

With four mail.app stalls. Period.
I hope this will be solved in Tiger’s version of mail.app.