Returned mail: Data format error

The original message was received at Mon, 18 Jul 2005 15:12:33 +0200
from lists.plug.phoenix.az.us [43.25.160.112]

----- The following addresses had permanent fatal errors -----
<lurker@lists.plug.phoenix.az.us>

----- Transcript of session follows -----
... while talking to mail server 101.103.199.164:
>>> DATA
<<< 400-aturner; %MAIL-E-OPENOUT, error opening !AS as output
<<< 400