I'm not the best with setting up the mail feature in this fashion - but you might also want to give something like this a try, although it probably won't make a difference.
<?php
error_reporting (E_ALL);
$retval = ini_set("SMTP", "mail.myISP.com");
if (!$retval) { print "Could not set SMTP!<br>\n"; }
$retval = ini_set("sendmail_from", "
youName@YourAddr.com");
if (!$retval) { print "Could not set sendmail_from!<br>\n"; }
..... Rest of script ... ?>
This basically just specify's those changes that where made to the ini file again - but inside the script instead of in the actual file.
I had to use something like this on one IIS server - but that was for a different reason.