If you are looking to configure an additional port for SMTP connections on Postfix, here are the steps you should follow:
The following process applies to Plesk 9x & 10x versions. For Plesk version 11.x, the alternate SMTP port 587 is open and you may edit your email client to use this alternate port.
1) Add the following line to /etc/postfix/master.cf
<IP_Address>:<port> inet n - n - - smtpd
Where <IP_Address> is the IP address of your server and <Port> is the additional port that you wish to configure
eg: 188.8.131.52:2025 inet n - n - - smtpd
2) If Postfix is configured to use Postfix Before-Queue Content Filter the add the following as well to the above line
eg: <IP_Address>:<port> inet n - - - - smtpd -o smtpd_proxy_filter=127.0.0.1:10025
3) reload Mailmng
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