Nasty Exchange problem on a Windows 2003 server

We have finally solved a nasty MS Exchange problem that was present on our Windows 2003 server.

Symptoms: Outgoing mails were 'collected' in a SMTP queue for about one week, although our SMTP server settings were correct and our IT's mailrelay was working. Mails to the mailrelay worked.

In fact, the IT-department had blocked one of our Server IP addresses, so that no internet out connection was present: Normal webpages and even IP-connections to the rest of the world were blocked - but only for one particular address. The connection to our mailrelay was not blocked.

It seems as if MS Exchange validates email addresses before sending them off by it's virtual SMTP server. After reconnection of the IP address Exchange started to fire immediately.