Since the ISP switch yesterday, no mail has left or arrived my mailserver.
It seems my ISP, per default, blocks all in- and outgoing traffic on port 25, which kind of makes sense when you take the typical customer in consideration who neither knows about this port, nor cares about setting up a mail server - but still is prone to be infected by spam-mailing trojans. But is completely friggin' STUPID when they know (from numerous conversations) that I'll be setting up a mail-server on the line.
It's a holiday here, today, so the support lines open late. I'll be their first caller!!
To be updated !