The biggest problem of all is Hotmail, and I still didn't find the "trigger" that marks them as SPAM.
For now I use PHPMailer (SMTP-auth) and it works awesomelly good on Yahoo, Gmail, AOL, stand-alone mail servers except Hotmail!
I sent them using the stardard and valid procedure (including valid headers to, from, reply-to with and without names), using both text or rich-text modes, but still hotmail will always refuse them as valid. As mentioned above, they are marked as spam only on hotmail.
This is a general problem among all people who use SMTP-auth with PHP.