Show the account specific settings in Enigmail in Tools > Account settings > OpenPGP security. Check that Enigmail is activated for the e-mail account and change the signature and encryption settings according to your preferences:
- Sign non-encrypted messages by default (1) signs all unencrypted messages with your key. Using this setting is a matter of preference.
- Sign encrypted messages by default (2) signs all encrypted messages with your key. This setting should be activated.
- Encrypt messages by default (3) encrypts all messages that you send. It is not worth keeping this setting activated if not everyone, or almost everyone, that you send e-mails to are GnuPG/PGP users.
- Use PGP/MIME by default (4) is an alternative way of presenting an encrypted message or signature to the e-mail program. Many e-mail programs do not recognise this format, so it is not worth using it unless the transmission of encrypted messages is not a problem.
If the e-mail address linked to your key differs from the e-mail address in Thunderbird, you cannot select the correct key based on the address. By clicking Select key you can tell Enigmail which key to use.
When you are writing a message, you can quickly change signature and encryption options for the message by clicking OpenPGP in the message composition window.