Send email using Postfix (MTA) Mail Transfer Agent via Gmail Relayhost

Emails sending with Postfix (MTA) Mail Transfer Agent via Gmail relay host. E-mails are delivered via Gmail account, the clients in the local network use Postfix as a local SMTP gateway. This tutorial shows how to do this.

In situations like these, for example, multifunctional devices, or apps that do not support authentication via Mail Submission and STARTTLS (port 587), e-mails can send via local Postfix MTA without need to log on to a mail server.

If CentOS Linux is used, the required packages are installed as root.

yum update && yum install -y postfix mailx cyrus-sasl cyrus-sasl-plain

On a Debian and Ubuntu, the package installation is as follows.

apt-get update && apt-get install -y postfix mailutils

The Postfix configuration is to be edited by opening the file /etc/postfix/main.cf

mynetworks = 127.0.0.0/8 192.168.1.0/24
myhostname = 12.34.56.78.dynamic.yline.res.cust.isp.net
mydestination = $myhostname, localhost.$mydomain, localhost
inet_interfaces = all
inet_protocols = ipv4
relayhost = [smtp.gmail.com]:587
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_security_options =
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt

Enter the local networks in mynetworks. As myhostname the PTR resource record that can be resolved on the Internet for determining the PTR entry, the following command can be executed in the terminal, as in the following example, or here.

$ curl -s ifconfig.co | xargs host
12.34.56.78.in-addr.arpa domain name pointer 12.34.56.78.dynamic.yline.res.cust.isp.net

Next, a /etc/postfix/sasl_passwd file is created with the following content.

[smtp.gmail.com]:587 mein@gmail.com:password

Instead of mein@gmail.com use your own Gmail account, with password the Google password.

Use postmap to create the file sasl_passwd as Berkeley DB.

postmap /etc/postfix/sasl_passwd

Hint. Run postmap after each change.

Now Postfix is restarted to activate the configuration.

systemctl restart postfix

Test the Postfix configuration with this line in the terminal.

echo "Here is a text." | mail -v -s "Test subject" -r mein@gmail.com andere@domain.com

The mein@gmail.com be the valid Gmail address, and if andere@domain.com, enter a valid recipient email.

Note. This Google Account requires disabled settings under Security – Sign in to Google – go to Security Verification and set two factor Off, and access to the Google Account when accessed by less secure apps must be On.

Postfix logging with syslog in the log file /var/log/maillog in which you can check whether the e-mail was sent and accepted by smtp.gmail.com.

tail -25 /var/log/maillog
mailq

Use mailq to check the queue for any rejected mails.

If you find authentication errors in maillog, the Google Account security settings must be checked.

status=deferred (SASL authentication failed; server smtp.gmail.com[108.177.126.108] said: 534-5.7.9 Application-specific password required. Learn more at?534 5.7.9

Hint. It can happen that after several sending attempts, then rejected (bounced) mails are in the queue, the command mailq gets them, with postsuper -d you remove mails from the queue.

postsuper -d ALL