Domain – The benefit of different SIP and SMTP domains

I believe this is what you want.

http://www.postfix.org/ADDRESS_REWRITING_README.html#generic

Example:

/etc/postfix/main.cf:
    smtp_generic_maps = hash:/etc/postfix/generic

/etc/postfix/generic:
    his@localdomain.local       hisaccount@hisisp.example
    her@localdomain.local       heraccount@herisp.example
    @localdomain.local          hisaccount+local@hisisp.example

So, if you have postfix installed and configured already.

Open main.cf and add the generic maps setting above.

Then open the generic file and add @example.com @example2.com or whatever settings you need for your environment

This is a hash file so when you are done run postmap /etc/postfix/generic

Then restart postfix.

UPDATE

It looks like this is actually what you are after.

edit the master cf file and change this line

smtp inet n - n - - smtpd

To:

localhost:smtp  inet n - n - - smtpd
ipaddress1:smtp inet n - n - - smtpd
ipaddress2:smtp inet n - n - - smtpd -o myhostname=hostname2
.
.
.
ipaddressn:smtp inet n - n - - smtpd -o myhostname=hostname3

restart postfix

Obviously this will require you to setup multiple IP address, one for each domain and setting up the reverse DNS etc. I believe this is the only way to do this.

Solved by checking applogs. Seems like a misconfigured PBX, which didn't want to reply on the correct port.