Postfix relay on MacOS High Sierra (and later)

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

Line 657 (or thereabouts) contains a inet_protocols directive.

Change it from:

inet_protocols = all

to:

inet_protocols = ipv4

And add the following:

# Customizations
# Disable backwards-compatibility mode -- not needed.
compatibility_level = 2

#Gmail SMTP
relayhost=[smtp.gmail.com]:587

#Enable SASL authentication in the Postfix SMTP client.
smtp_sasl_auth_enable=yes
smtp_sasl_password_maps=hash:/etc/postfix/smtp_sasl_passwd
smtp_sasl_security_options=noanonymous
smtp_sasl_mechanism_filter=plain

#Enable Transport Layer Security (TLS), i.e. SSL.
smtp_use_tls=yes
smtp_tls_security_level=encrypt
compatibility_level = 2

A couple notes:

• Regarding relayhost, enclosing smtp.gmail.com in square brackets prevents Postfix from doing an MX lookup on the hostname. Google explicitly tells you to use smtp.gmail.com in their docs, so no need to do an MX lookup on it.
• Your Gmail password is saved in a hash map named smtp_sasl_passwd. We’ll create it next.

Create smtp_sasl_passwd in your favorite text editor. The format should follow this pattern:

[smtp.gmail.com]:587 username@gmail.com:password

Replace the username with yours, and use an App Password. Google will fail your authentication attempt if you try it with your normal Google password.

Once that’s done, I like to set permissions on the file to owner read/write only, and then generate the hashmap.

sudo chown root:wheel /etc/postfix/smtp_sasl_passwd
sudo chmod 600 /etc/postfix/smtp_sasl_passwd
sudo postmap /etc/postfix/smtp_sasl_passwd

Then, start up Postfix:

sudo postfix start

You can verify that Postfix has started correctly with nc:

    nc localhost 25
    220 hostname.local ESMTP Postfix

Try pointing any applications to use the local smtp daemon, send a message, and you should be all set.

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