Server keeps sending out spam

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I've had issues with this before and I don't think there was ever a clear resolution to it.

http://www.pagasa.net/test-smtp-server/

I ran the SMTP test with the link above and it gets to step 17 and states that we are an open relay:


Test 17/28 (I blocked out my information for security purposes)
>>> RSET
<<< 250 2.0.0 Resetting
>>> MAIL FROM: <test@[xx.xxx.xxx.xxx]>
<<< 250 2.1.0 test@[xx.xxx.xxx.xxx]....Sender OK
>>> RCPT TO: <"test@spam.com">
<<< 250 2.1.5 "test@spam.com"@mydomain.com

Host is an open relay !

CPU time: 0ms - LA: 0.45 - 60 SMTP requests
Address: xx.xxx.xxx.xxx - Reverse: mail.mydomain.com
xx.xxx.xxx.xxx is blacklisted by Spamhaus
ESMTP: yes - TLS: no - AUTH: yes - VRFY: no - MTA: Microsoft
Average time : 0.14sec - Slowest : 0.27sec - Fastest : 0.14sec



I've gone through my SMTP settings and from what I found online made necessary changes, but it still says we are an open relay. The settings I have are currently:

Access Tab - Authentication

Anonymous, Basic and Integrated are checked

Users button - No groups or usernames listed

Access Tab - Relay

Only list below selected - No computers listed in box (Allow all computers option is unchecked)

Users button - No groups or usernames listed



Is there something else I'm missing? Can someone let me know if there is anything else that needs to be done to securely lock our server from sending out spam? I very much appreciate any help!

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Here is some information I got back from one of the blacklist settings:

Return-Path: <RichieSummerton3776@mydomain.com>
X-Original-To: evojydopou2174@SPAMTRAP.INVALID
Received: from mydomain.com (mail.mydomain.com [xx.xxx.xxx.xxx])
by mx.selfip.biz (Spamtrap) with ESMTP
for evojydopou2174@SPAMTRAP.INVALID; Wed, 13 Oct 2010 15:24:34 +0200 (CEST)
From: "Best-quality Pfizer" <RichieSummerton3776@mydomain.com>
To: evojydopou2174@SPAMTRAP.INVALID
Reply-To: RichieSummerton3776@mydomain.com
Subject: Mr. evojydopou2174, exclusive deal for you. The it equilateral
Mime-Version: 1.0
Content-Type: text/html; charset=iso-8859-1
Content-Transfer-Encoding: 8bit

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