SMTP relay through Fastmail from Postfix on macOS Mojave

When my Mac laptop tries to send me email — the output of a cron job, for example — by default it ends up in a local mailbox file that I never check. I want the mail to get to my regular email account, but I don’t want it to relay that message through whatever random ISP I might be connected to at any given time, or over the open internet. It’ll likely fail, it’s not secure, and there are better ways.

Instead, I relay all outgoing mail sent from macOS through my email provider, Fastmail (affiliate link). This especially makes sense since most of the email from my OS is going straight to my inbox hosted at Fastmail.

The notion of relaying email through a specific provider is built in to the Postfix mail transport agent that comes with macOS, so in theory it’s not a big deal to set up. In reality, I’ve found it to be a somewhat fragile configuration, and rarely does it survive a macOS upgrade or switch to a new computer. So I’ve come to document it pretty heavily for my own reference. I recently went through the process again, so thought I’d write it up here in case it’s helpful to others.

Most of these steps are derived from this nice compilation of steps that applies to macOS Sierra through Mojave.

  1. Edit /etc/postfix/ and add these lines to the end:

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