$ sudo apt-get update
$ sudo apt-get install heirloom-mailx 

heirloom-mailx is installed in your system.

Make ensure the heirloom-mailx package were installed using the commands given below,

$ sudo dpkg-query -l | grep heirloom-mailx *

You will get with heirloom-mailx package name, version, architecture and description in a table

heirloom-mailx package basic information: