So, a day later I'm back to Postbox now. Here's what broke it for me this time. It's hard to believe but these things are indeed still unfixed:
* The message list can not scroll to bottom. Neither on startup, nor on arrival of new messages. That bug[1] is open since 2010(!).
* The unread count and new message notification can not be disabled for specific IMAP-folders. Since my spam folder is a regular IMAP-folder (managed by spam assassin) Thunderbird insists on notifying me about every new Spam mail I get. Uhm, no thanks.
* The message list can not scroll to bottom. Neither on startup, nor on arrival of new messages. That bug[1] is open since 2010(!).
* The unread count and new message notification can not be disabled for specific IMAP-folders. Since my spam folder is a regular IMAP-folder (managed by spam assassin) Thunderbird insists on notifying me about every new Spam mail I get. Uhm, no thanks.
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=539468