Most university networks block port 25. Of course to prevent spamming. Makes sense, as most mail servers listen on port 25. Of course they also block me in using my mail-server. Stupid! That's why my mail server now also listens on port 8025, which gets through. So I can contact my own mail server.

It's as simple as adding the following line to Postfix's master.cf:

8025 inet n - n - - smtpd

And of course change the setting in your mail client :)