Notes on Postfix configuration and tricks.

I currently run my external mail on a Google Apps account. As a simple solution to allow me to send mail from several Linux systems, I have configured Postfix to use a specially configured Gmail account as a pseudo mail relay.

To do this you really only need to make a few configuration changes to the default Postfix main.cf file that ships with the Debian package.

# See /usr/share/postfix/main.cf.dist for a commented, more complete version

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no
delay_warning_time = 4h
readme_directory = no
myhostname = localhost
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = $myhostname
mydestination = localhost, localhost.localdomain, localhost
relayhost = [smtp.gmail.com]:587
mynetworks = 127.0.0.0/8
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = loopback-only
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl/gmail-passwdmap
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile =  /etc/postfix/gmail-cacerts.pem
smtp_use_tls = yes
smtp_tls_loglevel=1
smtp_tls_security_level=encrypt
compatibility_level=2

You can retrieve the current Google CA certs from this page: https://support.google.com/a/answer/6180220?hl=en

Store these certs as /etc/postfix/gmail-cacerts.pem.

You will also need to generate an app password to use with the account in Gmail.

https://support.google.com/accounts/answer/185833?hl=en

You can then configure this username and password in /etc/postfix/sasl/gmail-passwdmap accordingly:

[smtp.gmail.com]:587	my-smtp-relay@example.com:abcdefghijklmnop

Where *my-smtp-relay@example.com* is the Gmail account you will use and *abcdefghijklmnop* is your app password. You will also need to use postmap to create the Postfix binary map table for this file when you create or modify this file:

postmap  /etc/postfix/sasl/gmail-passwdmap