Background
By using this 'Pipe' fetchmail mechanism, mails are delivered to user mailbox as and when they arrived. Unlike the cron mechanism which runs at certain time, this method depends on when the mail been received by the SMTP and processed by the MTA (Mail Transfer Agent). Using Pipe mechanism, mails seems to be delivered instantly.
Make sure the file ../system-files/mx.php has 755 permission. If not, chmod the file to have 755 permission.
Your server using Qmail?
If your server is using Qmail, you need to specify a special flag in /system-files/mx.php
Open /system-files/mx.php file and find the following statement (around line #23):
define('__QMAIL__', 0);
change the value zero (0) to 1 to indicate that your server is using Qmail. Example :
define('__QMAIL__', 1);
Save the file.
Integration with Control Panel system
cPanel 9.x
From your cPanel screen, you will need to set the default address for your domain, so all unrouted mails (email not sent to known POP3 accounts) are sent to SocketMail. Click the Mail link from the main page. Click Default Address on the resulting page. Then click Set Default Address .
Select the domain that you want SocketMail to handle. In the input field, enter a pipe (the | character) followed by the full path to your mx.php file. Example:
| /usr/bin/php -f /path/to/system-files/mx.php |
Other control panel system
From customer holden, the way to set-up Pipeh mechanism from SSH/Telnet.
I wasnt able to setup the PIPE using the link provided, because I dont have cPanel. But I did find a nice little guide on setting it up with Sendmail via Telnet/SSH. Found it on other forums of all places

So I dont know if this has been posted before or not.. but just in case .. here's how ya do it...
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Firstly, you need to create an alias in sendmail to socketmail. You can do this by editing /etc/aliases and adding:
socketmail: |/path/to/system-files/mx.php |
or
socketmail: | php -q /path/to/system-files/mx.php |