Fix for MyBB emails going to spam box

If all of your emails, sent from php mail, or sendmail, are being put straight into the spam box, the way I fixed it was by changing line 318 in inc/class_mailhandler.php from :
"$this->headers .= "X-Mailer: MyBB."{$this->delimiter}";"
to:
$this->headers .= "X-Mailer: PHP/".phpversion()."{$this->delimiter}";

Now all of the mail, whether it be a user emailing another user, or a mass email, will go to the inbox, rather than the spam box.