Hi,
finally found some time to look into your issue and I have to say that it works as expected.
The only difference I can see between your code and my is that you use both <text> and <html> in one mail. If you do that then text will be visible as clear text not html and html stuff will be probably attached to it (at least that is what I observed during my tests) - as you described in your post as well.
Here is my mail node with your html message:
<mail name="mailtestmail" g="116,147,80,40">
<to addresses="your.email@gmail.com" />
<subject>mail</subject>
<html>
<body>
<a href="www.google.it">here</a>Ê
</body>
</html>
<transition name="to java1" to="java1" g="-47,-18" />
</mail>
I use google as smtp server over SSL.
Tested on jBPM 4.3 and 4.4.