Email in PHP

[nsd201]

okay...what i am creating is a newletter subscription form that emails out the newsletter. the confirmation email comes back to me, but only if i do the email address that ends with my domain name. Example:
email address = [e-mail]
site is mydomain.com

if i put an email address other than @mydomain.com the email is not sent.

I have been reading up on some smtp stuff, but i am confused. i want to be able to use mydomain.com to send out the email to the list. The server is a linux redhat server. Any ideas???

[planetsim]

Can we see some code.

[nsd201]

ini_set("sendmail_from","[e-mail]");
ini_set("SMTP","mail.mydomain.com");

$subscribemail = "Thank you for subscribing to the newsletter.";
$submailheaders = "From: $lettername subscription form\n";
$submailheaders .= "Reply-To: $youremail\n";

mail ($email,$lettername." subscription",$subscribemail,$submailheaders);

from the ini part i was able to send email but now the problem is that i can send mail to yahoo, hotmail but not to [e-mail]

I am wondering if it has something to do with the extra period. All other accounts receive email.