Sendmail in ubuntu over PHP

Was coding PHP to send mail to a user on form submission:

This was the code:

$to = "mail address";

$subject = "This is the subject";

$message = $_POST["arg1"] . $_POST['arg1'];

mail($to,$subject,$message, " -fmail@domain.com");

The code ran without errors but no mail appeared.

I installed sendmail:
sudo apt-get install sendmail

and tried a mail:
echo "mail out" | sendmail -v mail@domain.com


Now the problem was that sendmail tried to send via localhost:
Connecting to [127.0.0.1] via relay...

So, over to the sendmail config:  sudo vi /etc/mail/sendmail.mc
And added the row: define(`SMART_HOST',`esmtp:mail.domain.com')dnl
 

Restarted sendmail: sudo service restart sendmail

And BOOM! Mail recieved on form submission.