I set up git send-email following Drew DeVault's instructions https://git-send-email.io/#step-1
Pretty easy to follow as I'm using Fastmail, with only one gotcha.
I am using a custom email domain with Fastmail and I set that up in git. git@sillybit.domain.org
But upon trying to email my patch I got
5.7.0 Incorrect username or password.
I mistakenly tried to use my git email as the smtpuser too when I needed to use my actual fastmail user account at user@fastmail.com Makes total sense in hindsight.