Every once in a while, I wipe and re-install my laptop. When I do that, I invariably have to remember to add smtp relaying via GMail to my local postfix configuration. Since it’s something that I do so rarely, I’ve given up on trying to remember.

So, for posterity, assuming you’re using Gmail, and on MacOS High Sierra (nb: This still works on Catalina too):

In /etc/postfix, edit main.cf, make the following edits