Sending mail problem via php mail()

Question

I have a PHP server  with Apache 2 and PHP 4 and 5 and MySQL
I have a very very simple  mail script that only works with @gmail.com, hotmail.com etc
But  say if I had an email address for my company "[email protected]" it  does not go through.
Is this a PHP issue or a Server issue?
How  can I fix this as my customer is being an Arse!
mail("$mail1","Thank you for registering with $sitename",
"You have registered this email address at $sitename \n
Email: $mail1
Password: $pass
Date/Time: $date","FROM:$adminaddress");

Answer 1

First, you need to talk to your host, and have them install PEAR::Mail, and have them put the Mail.php path into the php.ini include path directive.
Here is a function I was using some time ago, before I switched to Zend Mail.
You need to edit / add the BOLD parts
<?php
require_once("Mail.php");
function sendEmail($to_mail, $to_name, $from_mail, $from_name, $subject, $body){
     $from = $from_name." <".$from.">";
     $to = $to_name." <".$to.">";
     $recipients = $to_mail;
     $headers["From"] = $from;
     $headers["To"] = $to;
     $headers["Subject"] = $subject;
     $headers["Reply-To"] = $from_mail;
     $headers["Content-Type"] = "text/plain; charset=UTF-8";
     $headers["MIME-Version"] = "1.0";
     $headers["Return-path"] = $from_mail;
     $headers["X-Priority"] = "3";
     $headers["X-MSmail-Priority"] = "Normal";
     $smtpinfo["host"] = "YOUR MAIL SERVER HERE";
     $smtpinfo["port"] = "25";
     $smtpinfo["auth"] = true;
     $smtpinfo["username"] = "YOUR EMAIL LOGIN USER HERE";
     $smtpinfo["password"] = "YOUR LOGIN PASSWORD HERE";
     $mail_object =& Mail::factory("smtp", $smtpinfo);
     $mail_object->send($recipients, $headers, $mailmsg);
?>
To send email:
<?php
sendEmail($mail1,
$recipient_name,
$from_mail,
$from_name, "Thank you for registering with $sitename", "You have registered this email address at $sitename \n
Email: $mail1
Password: $pass
Date/Time: $date","FROM:$adminaddress");
?>