Rickard, I've run across the same thing. I know you don't have access to a qmail server (as you said above), but I was curious if you'd found out anymore about the issue as yet?
Here's a copy of the error that qmail spits out to users:
Error: Unable to send e-mail. Please contact the forum administrator with the following error message reported by the SMTP server: "451 See http://pobox.com/~djb/docs/smtplf.html. ".
Hopefully the description in at the address will help.