Enable/Disable encryption to all recipient(s) before sending. User is notified, if encryption fails.
If Display selection when necessary is set in Preferences -> Key Selection tab, a list of keys will pop up if there are addresses in the list of recipients for the message for whom you have no public key.
If Never display OpenPGP key selection dialog is set in Preferences -> Key Selection tab, and there are addresses in the list of recipients for the message for whom you have no public key, the message will be sent unencrypted.
If you know the recipient(s) can read mail using the PGP/MIME format, you should use it.
This feature is dependent on the settings in Preferences -> PGP/MIME tab being set to Allow to use PGP/MIME or Always use PGP/MIME.
If there is a failure when actually sending mail, such as the POP server not accepting the request, Enigmail will not know about it, and the encrypted message will continue to be displayed in the Compose window. Choosing this menu item will undo the encryption/signing, reverting the Compose window back to its original text.
As a temporary fix, this option may also be used to decrypt the quoted text when replying to encrypted messages. Enigmail should automatically decrypt the quoted message, but if that fails for some reason, you can use this menu item to force it.
Further help is available on the Enigmail Help web page