After some configuration sending mail works again
Everything worked smoothly on the new network of onsneteindhoven.nl except sending mail. Receiving was not a problem. My guess is that all outgoing mail on port 25 (SMTP) is blocked except to the mail server of onsnet.nl
After some googling I found a few sites with instructions for using the onsnet.nl mail server as a relay using sendmail. It was relatively painless. Approx. 2 hours to get everything working. Best thing is that everything works just as it did with the xs4all network.
If you use FreeBSD, the attached text may be of help. YMMV !
Update on dynamic IP address: I've requested a static IP address. Costs 25 euro's but at least saves me some headaches with changing the DNS records by hand or automagically. Update: onsnet informed me that by using the fixed IP address 'normal' sending of mail should be possible. I haven't tried it yet (the relay method still works well) but I guess it will work (the gateway IP is different for a fixed address).
This port blocking is not done just to annoy me, but to block the outgoing mail of all these worm infected pc's on the ISP's network. It's sad that this is necessary. At Carnegie Mellon all pc's connected to the network were 'scanned' twice a day. I'm not sure what the scan exactly did (a portscan ?) but insufficient protected pc's were disconnected automatically. This meant you had to have a quite up to date win XP to be able to use the university network. I guess in the future ISP's will start to do the same. Get those zombie's of the net !