Install following global, if you want to use your executable. Otherwise you can install it in an virtual environment.

pip3 install prompt_toolkit
pip3 install pyaescrypt

Go to /usr/bin/ and create a file called e.g. mail It's content should be following:

#!/bin/bash
cd /path/to/this/sendmail/
python3 tui.py

To add your username and password for your email account, as well as the server data, run the file setup.py. For example:

cd /path/to/this/sendmail/
python3 setup.py

If you don't give some of the parameters, it'll use default values.

If you have forgotten your password for this mail-client, you have to do the setup again.