Sending email via Gmail relay through postfix

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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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