Install the package.
Sharing the public key
To distribute the public key one may upload it to a keyserver.
Encryption does not always work properly with emails containing HTML. It is best to use plain text by choosing Options > Delivery Format > Plain Text Only in the new email window.
Once the email is finished it can be signed through the OpenPGP menu.
Assuming that the email was encrypted properly, just trying to open it should result in a pop-up asking to type in the keyphrase.
Blank draft messages
If after upgrading tov2.1 your saved draft emails have gone "blank" and show a pink bar displaying:
Enigmail Error - no matching private/secret key found to decrypt message; click on 'Details' button for more information
or, when you have a "Write" window open you repeatedly see a popup window with:
The email address or key ID 0xkey_id cannot be matched to a valid, not expired OpenPGP key. Please ensure that you have a valid OpenPGP key, and that your account settings point to that key.
gpg --list-keys fails to show some keys that used to be there, see GnuPG invalid packet workaround.