Well..
After deinstalling and reinstalling the imap account it is working again.. but without any difference in the settings; strange.

Still, if anyone can help me, I would love to have an answer to my questions:
1. where can I find a logfile for the e-mail software?
2. different network (DNS) settings for different software?
3. how to flush DNS cache(s) on the N900?

thanks in advance