Pipe to program email duplication issue

Question

We have Cpanel with forwarding emails using "Pipe to Program" feature.

Forwarders example:

Email Address           Forward To  
[email protected]      |/home/user/forwarder/forward.php    
[email protected]      |/home/user/forwarder/forward.php

forward.php:

#!/usr/bin/php -q
<?php
$forwarders = [
    '[email protected]' => ['[email protected]'],
    '[email protected]' => ['[email protected]']
];

$recipients = $parseHeader['to'];
$to = [];
foreach($recipients as $recipient){
    if(isset($forwarders[$recipient])){
        foreach($forwarders[$recipient] as $forwarder){
            $to[] = $forwarder;
        }            
    }
}

//Send one email with emails in $to

Let's say I send an email to both '[email protected]' and '[email protected]' the above script will produce $to data:

[
    '[email protected]',
    '[email protected]'
]

We try to create a forwarded email that contains the same recipients but with the forwarded emails so the same email will be sent to '[email protected]' and '[email protected]'.

The issue is that both recipients receive 2 identical emails and if there is '[email protected]' it will be sent 3 times ..etc.

We tried to send an email with To: [email protected] and CC: [email protected] but the same thing happened.

Scenario:

1. An email is sent to '[email protected]' and '[email protected]'.
2. Forwarders trigger '[email protected]' and execute the script that sends an email to '[email protected]' and '[email protected]'
3. Forwarders trigger '[email protected]' and execute the script that sends an email to '[email protected]' and '[email protected]'

Is it possible to avoid this issue?

Answer 1

Like Alex says, you should really just forward the emails - most MTAs provide a mechanism for re-writing the sender (which is fairly essential nowadays). I'd also suggest you stop using cpanel, and unless you have a particular fondness for, and skills in Exim try Postfix.

The MOST likely cause is that this is running on Linux (you didn't say) and that you are invoking the next step of the journey via PHP's "mail()" function (again, you didn't say) without rewriting the original headers. In such a scenario, your code is simply invoking the command sendmail -t -i which determins the recipient from the headers in the email. Conversely with SMTP, the recipient is provided by the nagotiation before the mail headers and body are sent.

Either rewrite the headers and continue to use mail() or submit the forwarded message via SMTP or write your own wrapper around (e.g.) /usr/bin/mail