The most common reason for this problem is that your PHP/Perl program is not connecting to our mail server properly. In order to send e-mail, your program must specify a valid e-mail account user name and password and connect using SMTP Authentication.
Most popular programs support this, and you just have to login to their admin areas and change the Mail Server Type to "SMTP" and provide a valid e-mail address (User Name) and password.
If you are using a custom-made PHP program, you can download phpMailer (phpmailer.sourceforge.net) and edit your program to include it, as per their instructions, with this extra added bit:
$mail->Host = 'localhost';
$mail->SMTPAuth = true;
$mail->Username = $smtp_username;
$mail->Password = $smtp_password;
Where $smtp_username is a valid e-mail address on a domain you host with us, and $password is its password.
We have been forced to do this because of spammers who attempt to use our server to deliver their spam, which in the end hurts everyone.