A client's web hosting company has implemented security that requires specification of the mail host and source address. I looked through the existing code, but couldn't find such specifications, so I've added several lines to make this possible:

/* Specify your SMTP Server, Port and Valid From Address */
ini_set("SMTP","mail.yourdomain.com");
ini_set("smtp_port","25");
ini_set("sendmail_from","fromYou@yourdomain.com");

This is probably something that others would find useful, or if not included, at least documented somewhere.