Whilst I was looking through the syslog on my home server on an unrelated matter, I noticed some messages along the lines of:

 Jan  5 11:36:54 localhost postfix/smtp[29391]: 75858A424AC: SASL authentication failed; server smtp.gmail.com[64.233.184.109] said: 534-5.7.9 Appl ication-specific password required. Learn more at?534 5.7.9  https://support.google.com/mail/?p=InvalidSecondFactor f67sm99639626wmd.13 - gsmtp

The key part of the messages being SASL authentication failed and Appl ication-specific password required . Come to think of it, I hadn't had any emails from the server for a while. In fact, from around about the time I turned on two factor authentication.

After some research I managed to piece together what needed to be done to allow two factor authentication for postfix on Ubuntu. In case you haven't to set up gmail as a SMTP relay before, here's the steps to set it up.

Configure /etc/postfix/main.cf as follows:

 # Relaying Postfix SMTP via GMAIL
 relayhost = [smtp.gmail.com]:587
 smtp_sasl_auth_enable = yes
 smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
 smtp_sasl_security_options = noanonymous
 smtp_tls_CAfile = /etc/postfix/cacert.pem
 smtp_use_tls = yes

Add you gmail credentials to /etc/postfix/sasl_passwd

 [smtp.gmail.com]:587    MUYSER@gmail.com:MYPASSWD

Make sure perms of this file are secure, i.e.

 r-------- 1 root root    65 Jan  5 11:36 sasl_passwd

Compile and hash contents of /etc/postfix/sasl_passwd

 postmap /etc/postfix/sasl_passwd

Restart postfix

 systemctl restart posfix

You'll need to set up gmail to allow less secure apps. The procedure is here: https://support.google.com/accounts/answer/6010255

If you have enabled two factor authentication on your google account, these are the extra steps required:

Go to https://security.google.com/settings/security/apppasswords

 
 select app > Mail , select device > Other (Custom Name) > create a name > Generate

Add the application specific pssword generated to sasl_passwd replacing the one already there:

 run: postmap /etc/postfix/sasl_passwd

restart postfix and you're done.

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