I have created a hack for wp-mail.php that allows rich text emails to be processed. I call it wp-mail-rich. To use it you simply copy the wp-mail-rich.php file in the folder where WordPress is installed (where the wp-mail.php
file is) and use wp-mail-rich.php in place of wp-mail.php. If you have a cron job, just point it to the new file. To rollback just point it back to the old file.
I have also included some files that I used to test and develop this – in case anyone wanted to make further modifications. Instructions are in the readme.txt file.
I have only tested this with Gmail so it is possible that other rich text emails might not be exactly right coming from Yahoo, Hotmail, or your company mail server. Let me know if this is the case.