Mail delivery failed

Why would a new user on the email system using IMAP (probably) not be able to receive mail? When trying to send to him, I get disk quota exceeded. His account is only a few days old and this just happened this afternoon. I check in the email panel and he is showing 0/50MB used.

It appears that the disk space for this user is attached to the web disk space. I upped the limit, so it should work.