According to developer at http://www.phpguru.org/static/htmlMimeMail5.html

Author: Richard Heyes
Posted: 24th February 2005 12:52
John Lange:
> Richard, what is the license for this project?
> htmlMimeMail.php indicates it is copyright
> yourself which implies all rights reserved. But
> then again, it also says it was last updated in
> 2002!
>
> And I don't see any indication of a license
> mentioned on this site.
>

Consider the old version BSD license.

> I tried to email you directly using the address
> in mimePart.php (richard@phpguru.org) but that
> bounces.

Yup, that's not my email address. :-)