How to forward certain addresses only to another mail server with Postfix

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')

The behavior is described in http://www.postfix.org/local.8.html in the paragraph of Address Extension:

   For  example,  with  "recipient_delimiter  =  +", mail for
   name+foo is delivered to the  alias  name+foo  or  to  the
   alias  name,  to  the  destinations  listed in ~name/.for-
   ward+foo or in ~name/.forward, to the mailbox owned by the
   user name, or it is sent back as undeliverable.

But to be honest: Ditch that provider. Letting you configure mail forwarding only by .forward files is technology of the last century.