“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.
I don’t know either answer, since I do not use it.
Try here:
https://addons.mozilla.org/en-US/thunderbird/addon/enigmail/versions/