To make this more precise:

Whenever I click on an IMAP folder, TB! obviously reloads all headers from the server. For a big folder with 7000 messages this creates ~300KB traffic.

I tried setting synchronization to "No Synchronization" as well as "New Messages". It doesn't make any difference.

What is intereresting: When the computer is offline, TB! still tries to sync, but is able to display the folders correctly from its cache (and all message that are cached).

Can I disable that folder reloading?

UPDATE: After playing around with the IMAP settings I found out that the "Hitting folder fetch all flags every" is obviously what I've been searching for.

While the application is great, the documentation is really a pain and requires mainly a try and error approch...