Subject: Returned mail: see transcript for details

From: Mail Delivery Subsystem <[email-masked]>
Date: Tue, 23 Sep 2008 11:58:22 +0300 (EEST)

The original message was received at Tue, 23 Sep 2008 11:58:14 +0300 (EEST)
from [123.173.43.221]

   ----- The following addresses had permanent fatal errors -----
<[email-masked]>
    (reason: 550 Unknown user)

   ----- Transcript of session follows -----
... while talking to itc.paco.net.:
>>> DATA
<<< 550 Unknown user
550 5.1.1 <[email-masked]>... User unknown
<<< 503-All RCPT commands were rejected with this error:
<<< 503-Unknown user
<<< 503 Valid RCPT command must precede DATA