are you sure that you send a right formatted HTML mail...
have you set the MIME-Version?
here is an sample of a well formed MIME HTML mail:
Subject: Some Subject
Date: Wed, 17 Dec 2003 12:26:50 +0100
Message-ID: <E1E811923F2F054DB62E330FE5B3720544D7A7@sub.host.dom>
From: "Body, Some" <some.body@host.dom>
To: "Body, Any" <any.body@host.dom>
MIME-Version: 1.0
Content-Type: text/html;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML>
<HEAD>
<META HTTP-EQUIV=3D"Content-Type" CONTENT=3D"text/html; =
charset=3Diso-8859-1">
<TITLE></TITLE>
</HEAD>
<BODY>
<P><FONT SIZE=3D2 FACE=3D"Arial">This is a Test.</FONT>
</P>
<P><FONT SIZE=3D2 FACE=3D"Arial">Bye,</FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial">Some Body</FONT>
</P>
</BODY>
</HTML>
Have fun! :)