I’ve successfully installed vpopmail, qmail and courier today, after some hours of work on this server.
From now on you can send e-mail again to adresses, which won’t end up in /dev/null. (like bas.westerbaan@~)

To the users of the server: if you want an email-address, mail me.

While testing my smtp server whether it would receive incoming email properly I was harshly confirmed by it being spam instead of my test mail entering my mailbox first:

Hello Spam!
It’s pretty funny to see spam junk in an ancient text-mode mailing program.

If anyone experiences problems with any application not mailing you any verification as it should, please email me. It could very possibly be that the qmail-send isn’t configured properly to allow unauthorized local mails.

Update: More good news, the amount of RAM on my virtual server has increased to 120MB! Which means that my server won’t be swapping instead of working all the time anymore.