If you are not delivering outging mail directly but instead using a smarthost
, you can configure that directly in the PMG web UI
If that smarthost requires authentication, then AFAIK, you must add the auth config directly in postfix config /etc/pmg/templates/main.cf.in and a supplementary lookup file.
If you have more than one smarthost (e.g. sender dependent relayhost) you can configure multiple relay hosts. Similar to above with additon of a relay host lookup and additonal host username:pasword
lookups.
The postfix SASL howto above outlines it fairly well and the steps I mentioned above should work.