I really pity anyone that has to support PHP's Mail() function. It's one of the most buggy functions and gets worse with each new release of PHP. I've create many scripts over the years that used the mail() function. It was a support nightmare every time a new release of PHP came out. It hasn't improved with time.
I have made the decision to stop including support for PHP Mail() in PHPMailer Pro and the newly minted PHPMailer-Lite. I had already relegated the mail() support to the last possible transport class to use in the automated rotation.
The next release of my projects are due out any day now. The automated rotation will be in this order: SMTP, Sendmail, and IMAP. SMTP support is for both un-authenticated and authenticated.
Please note, you can also override the automated rotation. If you have a need to send via Sendmail, all you have to do is specify it when you issue the command to mail. That's as in:
$mail->Send('sendmail');
It doesn't have to be lower case, it can be all CAPS, CamelCase, or get funky and type in 'SeNdMaIL'.
Finalizing a few more tests before final release.