When running your own SMTP the real spam challenge is to avoid getting used by spammers. It is not to avoid getting targetted by spammers. Mind you, you don’t want spam in your inbox but a) that’s what filters are for and b) spammers (mostly) use confirmed lists of recipients, they don’t probe domains for addresses. When they do probe, they probe for access to relay, to use your server to distribute their junk. If they’re successful you’ll have far bigger problems than having to delete a couple emails a day (i.e. your IP and domain winding up on blacklists; also your ISP may disapprove).
That is the reason I have saved relaying until now. Opening yourself up to reciving unwanted mail is at lot less problematic than opening yourself up to having your server hijacked by spammers. Postfix is by default set to be rather restrictive in who it lets send out mail so while we have been messing about with receiving mail, the mail sending settings have stayed default and safe.
This will be a short post, as we will only get to know how to relay mail based on the mynetworks setting. ‘Relay’ is fairly synonynous with ‘sending’ but AFAICT it only refers to sending between SMTP servers. A user sends an email. An SMTP server relays it to another SMTP server. This post is about relaying from a local network.