携帯から私のPCにメールをもらい、すかさず返事をしたところ、以下のようなエラーとなってしまいました。
一体何が悪いんでしょう？

From: Mail Delivery Subsystem [mailto:[email protected]]
Sent: Wednesday, March 19, 2003 10:14 AM
To: [email protected]
Subject: Returned mail: Service unavailable


The original message was received at Tue, 18 Mar 2003 19:13:31 -0600
from corpmail.xxxxx.com [1xx.xx.xx.233]

----- The following addresses had permanent fatal errors -----
<[email protected]>

----- Transcript of session follows -----
... while talking to mfsmax.docomo.ne.jp.:
>>> DATA
<<< 550 Unknown user [email protected]
554 <[email protected]>... Service unavailable