Sending email via Gmail relay through postfix

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Sending email via Gmail relay through postfix

There is newer article at CentOS 8.x postfix send email through relay or smarthost with smtp authentication

Avoid use of less secure apps. Better option is to enable 2-step verification. Then create app passwords for apps that do not support 2-step verification / OAUTH etc.. See

Also note that Gmail is likely to stop support for less secure apps very soon (See https://support.excelmicro.com/index.php?/News/NewsItem/View/374/g-suite-switch-to-g-suite-apps-that-use-secure-oauth-access-as-password-based-access-will-no-longer-be-supported ). Most of emails sent via gmail using this technique neither seem to get delivered neither bounce back!!!.

To send email via Gmail relay through postfix use:

1. Configure gmail to use less secure apps using:
   1. Login into Gmail account and go to Settings -> Accounts -> Google Account Settings
   2. In "Google Account Settings" go to Sign-in and Security.
   3. In Sign-in and security change "Allow less secure apps:" to ON
2. yum -y install cyrus-sasl cyrus-sasl-plain cyrus-sasl-lib postfix mailx
3. Note that without cyrus-sasl and related packages error no worthy mechanisms found would be received while authenticating to Gmail via postfix. Refer https://serverfault.com/questions/325955/no-worthy-mechs-found-when-trying-to-relay-email-to-gmail-using-postfix
4. Set at least following in /etc/postfix/main.cf for mail system to work properly:

   myhostname
   mydomain
   myorigin
   inet_interfaces = all
   inet_protocols = ipv4

5. edit /etc/postfix/sasl_passwd and put something like

   smtp.gmail.com	<gmail-email-address>:<gmail-password>

6. Create hash postmap of sasl_password using:

   cd /etc/postfix
   chmod 600 sasl_passwd
   postmap hash:/etc/postfix/sasl_passwd

7. edit /etc/postfix/main.cf and after relayhost comment lines insert

   smtp_sasl_auth_enable = yes
   smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
   smtp_sasl_security_options =
   smtp_tls_security_level = may
   relayhost = smtp.gmail.com:587

8. service postfix restart
9. echo "Test email using postfix" | mail -s "Gmail relay test" saurabh@example.com
10. Look for successful email being sent or errors logs using:

    mailq
    tail -50 /var/log/maillog

    Logging into above configured "Less secure Gmail" account via browser also helps

To troubleshoot look at /var/log/maillog. In log lines look at value of ctladdr such as 'ctladdr=saurabh (501/501)'. Then look at /var/mail/saurabh or login as user saurabh and use 'mail' command to see error message sent by Gmail server.

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