$ sudo apt-get update
$ sudo apt-get install msmtp-mta 

msmtp-mta is installed in your system.

Make ensure the msmtp-mta package were installed using the commands given below,

$ sudo dpkg-query -l | grep msmtp-mta *

You will get with msmtp-mta package name, version, architecture and description in a table

msmtp-mta package basic information: