To avoid sending e-mails containing sensitive data unencrypted I strongly suggest you enable a confirmation dialog.

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Enigmail can show if the e-mail being sent will be encrypted or not -- and if, which keys will be used. This applies to both clicking the "Send" button or pressing Ctrl+Enter. Thus, you may disable the confirmation for the keyboard shortcut as you get a message box anyways which will avoid sending e-mails by accident.

Instructions

Head over to the Enigmail preferences from the "OpenPGP" menu and tick the corresponding checkbox.