rsmith wrote:9/10 ISP's do not allow relaying and you must authenticate on the mail server before it will send your message.
Setting the from address to a valid e-mail at your ISP's mailserver has nothing to do with SMTP authentication.
You are correct, however, in saying that many ISP's require the authentication. SMTP authentication, however, involves sending your username and password (just as if you were logging into your POP3 server to retrieve mail) which PHP's native mail() function does not support. If your ISP requires authentication, you must use some 3rd-party class to send mail (e.g. PHPMailer, XPertMailer, PEAR's mailing package, etc. etc.).