Emails are not sending what-so-ever

E-mails are not sending from the contact form or from the administration area of Zen Cart.

I am using Zen Cart on Windows Server w/IIS and the SMTP server is a different server on the same domain.

I have tried switching from smtpauth to PHP but neither are working.

However, when I switched to PHP and changed the test customer profile account's e-mail address to one on the same domain, it says the e-mail sends successfully but the e-mail is not received. But, other domains are not working, such as trying to send to a gmail account.

Any help is appreciated! Thank you!

Update:

It is sending e-mails using PHP to one address on the domain (eg. [email protected]) but it errors on another address (eg. [email protected])

Now I am really confused!

you could try to upload this at your webserver somewhere

<?php 
$to = "[email protected]"; 
$subject = "Hi!"; 
$body = "Hi,\n\nHow are you?"; 
if (mail($to, $subject, $body)) {   
echo("<p>Email successfully sent!</p>");  
} 
else 
{   
echo("<p>Email delivery failed…</p>");  
} 
?>

run that from your browser and a mail should be send (using the internal php mailer, edit the email ofcourse)

if you dont receive the email then there is something wrong with the settings in php.ini