Install gnumail.app-common by entering the following commands in the terminal:
sudo apt update sudo apt install gnumail.app-common
Description:
Mail client for GNUstep (common files)
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.). . This package contains the architecture-independent files; it is not useful on its own.
Homepage: http://www.nongnu.org/gnustep-nonfsf/gnumail/
Version: 1.2.3-1build1
Section: universe/gnustep