i know, but it surprised me that it also scans non-standard port-numbers … and even puts in the ‘clean’ message !

Well, I don't know mail proxies too much. Probably if you read the [b]MailScanner[/b] section in avast4.ini file (see 'Settings' on my signature) you will be able to 'redirect' avast and tweak its behavior. Hope this helps.
yeah, there is a very good description in the Help under "Manual setting of mail protection" ... works like a charm.