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Edit /etc/services file and declare another network service. This file maps services name to their underlying assigned port numbers and protocol types. For example add the following smtp2 service

...
smtp            25/tcp          mail
smtp2            2525/tcp          mail
..

You just have to choose a name, a port (here 2525) and a protocol (tcp).

Then in Postfix's master.cf file, add the smtp2 service declaration:

smtp      inet  n       -       -       -       -       smtpd
smtp2      inet  n       -       -       -       -       smtpd

Finally reload Postfix