The simple answer is that your mail clients are probably interpreting it as text/plain, and you will likely have to set the Content-type for the section. The Mail::Sendmail FAQ comments on this further:

How to send HTML mail?
Instead of sending your mail with a Content-type of "text/plain", you send a "multipart/alternative" MIME mail, where one part is "text/plain" (for clients which cannot display HTML) and the other is "text/html".

There are ways to simplify this if you are absolutely sure everyone can handle HTML email, which are also noted at that same link. Hope that helps.

Update: 18 Apr 2005
Fixed link to point to specific subsection of document.