$ sudo apt-get update
$ sudo apt-get install dovecot-pgsql 

dovecot-pgsql is installed in your system.

Make ensure the dovecot-pgsql package were installed using the commands given below,

$ sudo dpkg-query -l | grep dovecot-pgsql *

You will get with dovecot-pgsql package name, version, architecture and description in a table

dovecot-pgsql package basic information: