Fixing "501: Sender domain must exist" errors.

["Christopher R. Barry" <cbarry@2xtreme.net>]

My machine name is "HAL" and when sending email to many places I get
errors like:

From: <MAILER-DAEMON@HAL>
Subject: mail failed, returning to sender
To: cbarry
Date: Sun, 28 Mar 1999 11:23:32 -0800 (PST)
X-From-Line: MAILER-DAEMON Sun Mar 28 11:23:56 1999
Return-Path: <MAILER-DAEMON>
Received: from HAL by HAL
	via smail with bsmtp
	id <m10RL9g-000kI2C@HAL> (Debian Smail3.2.0.102)
	for <cbarry>; Sun, 28 Mar 1999 11:23:32 -0800 (PST) 
Message-Id: <m10RL9g-000kI2C@HAL>
Reference: <m10RL9T-000kBiC@HAL>
Lines: 57
Xref: HAL other:3609

|------------------------- Failed addresses follow: ---------------------|
 someone@somewhere.com ... transport smtp: 501 <cbarry@HAL>... Sender
domain must exist

I guess I've got to make the "HAL" a "2xtreme.net" by editing a file
in /etc but which one and how?

Thanks,
Christopher