$ sudo apt-get update
$ sudo apt-get install courier-authlib-postgresql 

courier-authlib-postgresql is installed in your system.

Make ensure the courier-authlib-postgresql package were installed using the commands given below,

$ sudo dpkg-query -l | grep courier-authlib-postgresql *

You will get with courier-authlib-postgresql package name, version, architecture and description in a table

courier-authlib-postgresql package basic information: