One of the things I'm sorry I didn't do earlier is setup postfix on my Mac, such that I'll be able to send quick emails (not to mention git patches) directly from command line.

As we all know, sending emails directly from your machine is a sure way to get yourself blacklisted as spam. So using an SMTP relay is pretty much required. But since my main email account is hosted on Gmail, and I want to be able to connect securely to Googles SMTP servers, this requires some configuration.

First thing's first, add your authentication details to the relay. If you're using Gmail, this goes like this, create a new file:

sudo vi /etc/postfix/relay_password

And add the auth details to it, just one line:

smtp.gmail.com:587 your_user_name@gmail.com:your_password

Next, we need to generate a lookup DB from these details:

sudo postmap /etc/postfix/relay_password

And make sure the relay_password.db file has been generated.

Now it's time to update the main.cf configuration file. You might want to keep a backup before you add the following changes. First, check that the line

tls_random_source = dev:/dev/urandom

exists in the file and is not commented out, this should be the case by default. Now here's the main logic which you can simply append to the end of the file:

relayhost = smtp.gmail.com:587

smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/relay_password
smtp_sasl_security_options = noanonymous

smtp_tls_security_level = may
smtp_tls_CApath = /etc/postfix/certs
smtp_tls_session_cache_database = btree:/etc/postfix/smtp_scache
smtp_tls_session_cache_timeout = 3600s
smtp_tls_loglevel = 1

The last thing we need to do is setup the root SSL certificate that Google uses, which is the Thawte Premium Server CA. First:

sudo mkdir /etc/postfix/certs && cd certs

Then, download the PEM file:

sudo wget https://www.thawte.com/roots/thawte_Premium_Server_CA.pem

Now we need to run a rehash on the PEM file:

sudo c_rehash /etc/postfix/certs/

And that's it! Give it a test run, and hopefully you'll receive an e-mail strongly authenticated and relayed from your Gmail account:

echo "Relay Test" | mail -s "Relay Testing" test_recipient@domain.com

As an extra added bonus, you might want to set your hostname to something more descriptive than mymachine.local by adding this line to the main.cf:

myhostname = some-domain-i-own.com