Sending email using php mail function - php

Question

I am sending emails using php's mail function. Some addresses in email clients such as aol, hotmail.com and msn are not receiving it. Gmail works fine! I find it strange as it can pass through Gmail but not others:
$semi_rand = md5(time());
$mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
$headers = "From: $from";
$headers .= "\r\nMIME-Version: 1.0\r\n" .
"Content-Type: multipart/mixed;\r\n" .
" boundary=\"{$mime_boundary}\"";
And then I am sending email using:
$ok = #mail($to, $subject, $message, $headers);
For diagnostic purposes $header string contains:
From: xxx#example.com
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="==Multipart_Boundary_xd31b2fcd6941ba77b38f866330c24944x"
and the $message string contains:
This is a multi-part message in MIME format.
--==Multipart_Boundary_x0dde39863d56158409aa962fc1dd9a3bx
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
The email message appears here
--==Multipart_Boundary_x0dde39863d56158409aa962fc1dd9a3bx
Content-Type: {"application/octet-stream"};
name="afile.pdf"
Content-Disposition: attachment;
filename="afile.pdf"
Content-Transfer-Encoding: base64
The attachment's contents here
--==Multipart_Boundary_x0dde39863d56158409aa962fc1dd9a3bx

Answer 1

Some addresses in email clients such as aol, hotmail.com and msn are not receiving it
Your code is not the best place to start investigating this - check your email logs and bounce messages in the first place. The most likely cause is that your email is being flagged as a spam. You could verify this by setting up a mailbox yourself and sending mesages to it. How to prevent your emails being flagged as spam is a question which has been asked and answered here many times.
I am not sure whether the header is correct
It would have been mkore helpful to see an example of the email and headers generated (trimed down to a sensible size).