php e-mail question

I have built a basic e-mail form, but it will only send to a yahoo account. I can get it to send
to a pop3 domain account like mark@neowav.com or a qwest account. It doesn't matter whether I go
online using horde or use download with express. The email just doesn't get there. But yahoo
receives it without a problem. Am I missing a required field?

EXAMPLE:
$mail_to = "$recipient_email";
        

        $mail_from = "$your_email";

        

        $mail_reply_to = "$your_email";

        $mail_headers = "From: $mail_from\r\nReply-to:
                        $mail_reply_to\r\n";
        $mail_subject = "$your_name has sent you an email";

        

        $mail_body = "Hi there";

        mail($mail_to, $mail_subject, $mail_body, $mail_headers);

Thanks,
Mark

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