Hi there, I've been having separate mail problems of my own, though I went the PHPMailer route, and it worked for me...
On the other hand I downloaded the PEAR Mail package and sifted through the code...
I'm not familiar with the mail() functions deep internals, though I've noticed capitalization in the PHPMailer internals for CC and BCC... while they were being passed to the mail() function...
Back when the mail function worked for me, the CC worked when capitalized ... I never tried it otherwise...
Try using
$headers['CC']=$cc; // capitalization of Cc to CC
I don't guarantee much at all, just that there are sometimes simple reasons to complicated problems... 😕