For all of the people struggling with getting PHP mail() sending to some, but not all, recipients...look no farther. Your solution has been posted here.
It appears PHP-generated emails don't play nice on Windows mail servers. So, when your mail goes to anyone who uses a Windows mail server (Comcast, gmail, and maybe aol), the recipients won't get it. UNLESS YOU ADD THE LINE:
<?
phpini_set("sendmail_from", "info@mydomain.com");
?>
This is a miracle! I have been scouring the internet high and low, and this is the ONLY thing that has worked! Now I can move on to the rest of the project!
Thank you, MeixiuLin! Thank you, thank you.