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 using like multifunctional devices, or apps that do not support authentication via Mail Submission and STARTTLS (port 587), then e-mails can send via local Postfix MTA without need to authenticate 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
Now edit Postfix configuration by opening the file /etc/postfix/main.cf
mynetworks = 127.0.0.0/8 192.168.1.0/24
myhostname = 220.127.116.11.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_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 record, hit the following command to run in a terminal, or click here.
$ curl -s ifconfig.co | xargs host
18.104.22.168.in-addr.arpa domain name pointer 22.214.171.124.dynamic.yline.res.cust.isp.net
Next, a /etc/postfix/sasl_passwd file is created with the following content.
Instead of firstname.lastname@example.org use your own Gmail account, replace password with your account password.
Run postmap to create the file sasl_passwd as Berkeley DB.
Hint. Run postmap after each change.
Now Postfix is restarted to activate the configuration.
systemctl restart postfix
Verify the Postfix configuration with this line in the terminal.
echo "Here is a text." | mail -v -s "Test subject" -r email@example.com firstname.lastname@example.org
Replace email@example.com with your valid Gmail address, and for firstname.lastname@example.org 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
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.
Note. To help keep your account secure, from May 30, 2022, Google no longer supports the use of third-party apps or devices which ask you to sign in to your Google Account using only your username and password.
status=deferred (SASL authentication failed; server smtp.gmail.com[126.96.36.199] said: 534-5.7.9 Application-specific password required. Learn more at?534 5.7.9
Note. after several sending attempts, then rejected (bounced) mails are in the queue remains, use the command
mailq to show them, ran
postsuper -d to remove deferred mails from queue.
postsuper -d ALL