Mails are not recevied

[KaramChand]

Hello,

We send our software upgrade details over email once a user has given his old order number and email address.

We generate the mail format using the following code:

[code]
                           $message  =     "Greetings,\n\nBlahBlahBlah\n\n";
               $message .=    "BlahBlahBlah\n\n";
               $message .=     "BlahBlahBlah\n";
               $message .=    "BlahBlahBlah\n\n";
               $message .=    "BlahBlahBlah";
               $message .=    "BlahBlahBlah\n\n";
               $message .=  "BlahBlahBlah\n\n";
               $message .=    "BlahBlahBlah\n\n";
               $message .=    "BlahBlahBlah\n\n";
               $message .=    "BlahBlahBlahn\n";
               $message .=    "BlahBlahBlah\n\n";
               $message .=    "BlahBlahBlah\n\n";
               $message .=    "Regards,\nKaram Chand";
            
               $headers .=  "Bcc: support_upgrade@xxx.com\r\n";

                           $myname  = "Karam Chand Support";
                           $myemail = "noreply@xxx.com";
                           $subject = "Blah Blah Blah - Download instructions and registration code";  
                          
                           $ret = mail( $email, $subject, $message, $headers );

[/code]

As you can see we send a BCC to us too.

We receive all the upgrade mails but sometimes some of our customers dont get it. The percentage is like 20% which is pretty huge.

What might be the problem?