Maximum length of MAIL FROM address

They're probably using BATV or some similar mechanism. It's a mechanism that filters out bouncebacks from messages not sent through that service. It exists so you don't receive bouncebacks when a spammer uses one of your emails as From and/or Return-Path.

The solution for that case is making the server use gmail to send, or use a Return-Path that doesn't go to gmail.

Best of luck,
João Miguel Neves

I am always reluctant to put services on ports that they don't usually live-- not because of a software deficiency, but because of people deficiencies. A sysadmin who inherits this setup has to be pretty good to track down the architecture of "where emails go when they bounce", or else your documentation needs to be pretty clear (and easy to find).

So, to answer your first question-- my suggestion is to have a separate server that handles bounces. This makes things nicely documented via dns, instead of buried in a config file for postfix.

If you choose to ignore that advice, utilizing the transport maps of postfix will allow you to do so. For example, adding this to main.cf:

transport_maps = regexp:/etc/postfix/transport

and using something like this in your transport file:

/bounce.*/      smtp:bounces.mydomain.com:8025

(don't forget to 'postmap /etc/postfix/transport' and 'postfix reload')