The most commonly used implementations of SMTP in most Linux distros are Sendmail and Postfix. Postfix is an open-source mail-transfer agent that was originally developed as an alternative to Sendmail and is usually set up as the default mail server.
We’ll show you how to figure out what version of the Postfix mail system is running on your Linux system.
Traditionally, popular options such as -v or -V or –version, depending on what the developer specifies, were used to display the version of an application or program installed (or running) in Linux, particularly on the terminal:
$ php -v $ curl -V $ bash --version
Type the following command in the terminal to find out what version of the Postfix mail system is running on your system. The -d flag causes the /etc/postficmain.cf configuration file to display default parameter settings instead of actual settings, and the mail version variable stores the package version.
$ postconf -d mail_version
$ postconf -d | grep mail_version
The above command is compatible with all Linux operating systems incl. CentOS, Debian, Fedora, and Ubuntu.