$ sudo apt-get update
$ sudo apt-get install php-horde-smtp 

php-horde-smtp is installed in your system.

Make ensure the php-horde-smtp package were installed using the commands given below,

$ sudo dpkg-query -l | grep php-horde-smtp *

You will get with php-horde-smtp package name, version, architecture and description in a table

php-horde-smtp package basic information: