https://enigmail.net/index.php/en/
What is Enigmail?
Enigmail is a seamlessly integrated security add-on for Mozilla Thunderbird. It allows you to use OpenPGP to encrypt and digitally sign your emails and to decrypt and verify messages you receive.
Enigmail is free software. It can be freely used, modified and distributed under the terms of the Mozilla Public License.
“Enigmail is free software. It can be freely used, modified and distributed under the terms of the Mozilla Public License.”
1. Can you choose, Email by Email to encrypt or not encrypt your messages. (I would assume that if I did encrypt an Email, the recipient would likely need Enigmail in their Email app/apps in order to decrypt an encrypted Email I sent them.)
2. Can you send and/or receive messages not encrypted, that is not encrypted at all.