Configure Postfix relayhost (smarthost) to send eMail using an external smptd

How to configure Postfix relayhost (smarthost) to send eMail using an external smptd

How do I configure Postfix MTA to send eMail using an external cloud-based SMTP server (with username: password) from a web server on Linux or Unix-like system? How do I configure an Ubuntu server and postfix as the relay server (smarthost)?

You can configuring Postfix MTA to use as a Smarthost i.e. routing all mails to a smarthost. A smarthost can be an external smtpd server provided by SendGrid, AWS, Rackspace, Google apps/Gmail, ISP or your own server in another data center. In this tutorial, you will learn how to install a Postfix smtpd and send email through ISP/Google Apps/SendGrid/AWS/Rackspace cloud email services with the help of the relay server.

Install postfix (if not installed on a web server)

Type the following command to install postfix:
$ sudo apt install postfix

$ sudo apt-get install postfix

Install pluggable authentication modules
You must install the libsasl2-modules for authentication purpose using apt command:
$ sudo apt-get install libsasl2-modules postfix

$ sudo apt install libsasl2-modules postfix

Configure myhostname

Edit /etc/postfix/, enter:
$ sudo vi /etc/postfix/

$ sudo nano /etc/postfix/

Set myhostname to FQDN as configured earlier (see fig.03):

myhostname =
Save and close the file.

Setup the relay server

Again open /etc/postfix/, enter:
$ sudo nano /etc/postfix/

Edit/Update it as follows:

# Enable auth
smtp_sasl_auth_enable = yes
# Set username and password
smtp_sasl_security_options = noanonymous
# Turn on tls encryption
smtp_tls_security_level = encrypt
header_size_limit = 4096000
# Set external SMTP relay host here IP or hostname accepted along with a port number.
relayhost = [YOUR-SMTP-SERVER-IP-HERE]:587
# accept email from our web-server only
inet_interfaces =

Save and close the file. Since you changed to inet_interfaces, stop and start Postfix, type:
$ sudo systemctl stop postfix
$ sudo systemctl start postfix

$ sudo systemctl restart postfix

Verify that TCP port #25 is in listing state on
$ netstat -tulpn | grep :25

Sample outputs:

(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
tcp 0 0* LISTEN –
Send a test mail using Postfix the relay server

The syntax is:

echo “This is a test email body.” | mail -s “Subject” -a “From:”
You verify that email sent or not using your own log file:
$ sudo tail -f /var/log/mail.log

$ sudo journalctl -u postfix

And there you have it, routing all mails to a smarthost hosted in cloud or your own data center from a web-server

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