$ sudo apt-get update
$ sudo apt-get install geany-plugin-sendmail 

geany-plugin-sendmail is installed in your system.

Make ensure the geany-plugin-sendmail package were installed using the commands given below,

$ sudo dpkg-query -l | grep geany-plugin-sendmail *

You will get with geany-plugin-sendmail package name, version, architecture and description in a table

geany-plugin-sendmail package basic information: