Install Thunderbird
sudo apt-get install thunderbird

Install gnupg
sudo apt update
sudo apt-get install gnupg

Install enigmail (Thunderbird gpg extension)
sudo apt-get install enigmail

Generate keys with gpg
See gpg

Select keys in Thunderbird
Open Thunderbird and apply keys:

  1. Navigate to Account Settings > [select account] > OpenPGP Security
  2. Select Enable OpenPGP support for this identity
  3. Select Use specific OpenPGP key ID and then Select Key