User unknown?

[Guillaume R.]

Re
To be more accurate here is a trace that I can found in
/var/mail/root. Seems to be a internal mail for my user (which never
receive any mail...)

The original message was received at Fri, 11 Nov 2005 19:34:32 +0100 (CET)
from root at localhost

   ----- The following addresses had permanent fatal errors -----
gnux
    (reason: 550 5.1.1 <gnux at localhost.my.domain>... User unknown)
    (expanded from: gnux)

 ----- Transcript of session follows -----
... while talking to [127.0.0.1]:
>>> DATA
<<< 550 5.1.1 <gnux at localhost.my.domain>... User unknown
550 5.1.1 gnux... User unknown
<<< 503 5.0.0 Need RCPT (recipient)

There is a lot of "mails" like that in this directory. Could anyone
explain me why? Indeed I cant find anything in the handbook about this
internal mailer daemon...
Thx