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

courier-authlib-userdb is installed in your system.

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

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

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

courier-authlib-userdb package basic information: