By default, the OpenPGP extension in Thunderbird (Enigmail) sends every encrypted mail as inline text, not as an attachment. Mutt as example does not recognize the encrypted mail but just displays the unreadable binary stuff.
This are the steps to send well-formatted encrypted mails with Thunderbird:
- Menu Extras, Accounts
- Make sure, the PGP/MIME checkbox is set
That's all. Now Thunderbird should send every encrypted text/attachment as a mime-attachment which can be recognized by other mail programs.