It seems my mailserver has been bouncing mails this morning.
My spam filtering setup was broken somewhere in the virus-scanning phase, as clamav was reporting errors. This caused all mail to be rejected. This should now be fixed (at least for now, as I disabled the clamav plugin on pythonfilter).
Also, someone had hacked into my ICQ account (which I hardly use anymore) and spammed all my contacts for some russian site. The password on my ICQ account has been changed, so this should no longer be an issue either.
Sorry for causing this mess