Warning: mail(): SMTP server response: 451 See http://pobox.com/~djb/docs/smtplf.html.
This error message indicates that you send email from PHP containing single linefeeds (\n) instead of real email linefeeds (\r\n).
You can read more about the problem here:
Usually you can fix this problem by making sure that before you call the mail() function you change all single \n to \r\n:
$ent = str_replace("rr", "r", str_replace("n", "rn", $ent));