More email sending problems.

Okay that worked, but now he's getting an error further on.

SENDING: HELO xxx.xxx.0.3
COMPUTER SAYS:250 smtpout.xxx.xxx.com Hello xxx.xxx.0.3 [xxx.xxx.xxx.xxx]
SENDING: MAIL FROM: <xxx@...;
COMPUTER SAYS:250 OK
SENDING: RCPT TO: <xxx@...;
COMPUTER SAYS:250 Accepted
SENDING: DATA
COMPUTER SAYS:354 Enter message, ending with "." on a line by itself
Sending Message Contents
Encoding file attachment...
--boundarystring
Content-Type: application/octet-stream; name=undo\temp_893_21060__gamestate.NWTc
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=undo\temp_893_21060__gamestate.NWTc

(encoded file contents not displayed)

--boundarystring--
COMPUTER SAYS:550 Administrative prohibition    <---------------ERROR
COMPUTER SAYS:221 smtpout.xxx.xxx.com closing connection

Any idea what's causing the "Administrative prohibition" error?