To relay our mail from a home/office server trough real mail server, that has all needed stuff like PTR, SPF, DKIM etc. stuff setup, the easies way is to 'login' to a free, open service like gmail, yahoo etc. server and send from our account there.
So, let's do this with postfix.
The EASY way to do is, if the provider supports submission protocol (port 587).
Then in main.cf we set a relayhost