non-existant users in my domain sending mail to users in mydomain

[tashi]

hi everybody,lately I'm experiencing some strange problem in my mail server.a non existent user in my domain is sending mail to my valid users in the domain.should this pose any security risk to my users and how do I solve it?...pls anybody on the forum.. help me out....

Return-Path: <non-existant user@mydomain>
Received: from swangdi (unknown [x.x.x.x])
	by mydomain (Postfix) with SMTP id E18CF14E4010
	for <non-existant user@mydomain>; Fri,  6 Aug 2010 14:35:21 +0600 (BTT)
To: swangdi@mydomain
Subject: My Best Photo
Date: Fri, 6 Aug 2010 14:46:24 +0100
MIME-Version: 1.0
Content-Type: multipart/mixed;
	boundary="----=_NextPart_000_0008_01C633DA.513F2E20"
X-Priority: 3
X-MSMail-Priority: Normal
X-Unsent: 1
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106
Message-Id: <20100806083524.E18CF14E4010@mail.mydomain>