Install gnumail.app by entering the following commands in the terminal:
sudo apt update sudo apt install gnumail.app
Description:
Mail client for GNUstep
GNUMail is a clone of NeXT/Apple's Mail.app application. It uses the GNUstep development framework (or Apple Cocoa, which is based on the OpenStep specification provided by NeXT, Inc.).
Homepage: http://www.nongnu.org/gnustep-nonfsf/gnumail/
Version: 1.2.3-1build1
Section: universe/mail