mail(): SMTP server response: 503

[alcibar]

Here's a little bit of code that doesn't work: What's the problem of this? I got this error:
Warning: mail(): SMTP server response: 503 No recipient(s). in C:\Xammp\xampp\htdocs\LoveSaloon\reg_one.php on line 133

Note: I'm using Xammp with MercuryMail.

$subject = "Your Membership!"; 
    $message = "Dear $firstname $lastname, 
    Thank you for registering at our website, <a href='http://www.mydomain.com' target='_blank'>[url]http://www.mydomain.com[/url]</a>! 

You are two steps away from logging in and accessing our exclusive members area. 

To activate your membership, 
please click here: <a href='http://www.mydomain.com/activate.php?id=$userid&code=$db_password' target='_blank'>[url]http://www.mydomain.com/activate.php?id=&#...036;db_password[/url]</a> 

Once you activate your memebership, you will be able to login 
with the following information: 
Username: $username 
Password: $random_password 

Thanks! 
The Webmaster 

This is an automated response, please do not reply!"; 

mail($email_address, $subject, $message, 
    "From: MyDomain Webmaster<[e-mail]>\nX-Mailer: PHP/" . phpversion()); 
echo 'Your membership information has been mailed to your email address! 
Please check it and follow the directions!'; 

Any help is greatly appreciated.

Thanks!

[drew010]

if its saying 503 no recipients, make sure that $email_address is actually set.