Sending MailFollow

When I try to send mail, I fill in the recipient, a subject, some text and try to send...

It says "Recipient not found"... Every time.

I tried sending to my friends, none of their char names work, I tried sending to my other chars, nothing... I tried sending to random people who were standing next to me at the mailbox, nope, "Recipient not found".

Am I missing something obvious?

Yes, all chars I tried sending to were on the same server
Yes, case was correct (not that I thought it would be case-sensitive)
Yes, I've tried sending from a number of my chars, all with the same result.

Maybe the WoW Post-Office banned me?