HTML Mail problem

If I send a HTML email using either mail() or pear mail to an email account of someone who is on my email server it does not get delivered right. But if I send a HTML email to an external email address it is fine. It also works fine with plain text email, only if you send a HTML format email. I have even setup pear mail not to use the mail function but link right to qmail's bin folder. I can also send HTML between users in HTML using Outlook fine, only if PHP sends do I have problem. I am using PHP 4.3.0 and qmail as my email server. Has anyone any idea of were to start looking for why this is happening. Again this only happen if it is a HTML format email and it is to an internal user, thus my email server routes it without leaving the server. Thanks, Mark.