Hi everyone.
I have made a strange experience. I have activated the RBL support and one the my RBL providers, spamhouse.org, is down. This effected every smtp traffic on my server and caused very strange behaviour. SMTP-debug shows network connection timeouts. Some Mails come in, some dont. Some users can send some mail but not all their mails. After I removed the entry from the configuration document, all back to normal.
Hope you find this helpful.
Cheers - Klaus