We've suspected for a while that we were banging our heads against a wall, now here's a possible explanation:
This is the Postfix program at host msxmx03.webde.de.
I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to
If you do so, please include this problem report.
You can delete your own text from the attached returned message.
The Postfix program <blackhole@msxmx03.webde.local>: unknown user: "blackhole"
Taken from a bounce message after sending a mail to the web.de postmaster.