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When Thunderbird (TorBirdy 0.2.1) downloads and saves an email locally it at adds X-Mozilla-Keys, if the user forwards those email as an attachment the X-Mozilla-Keys are still in the email (resp *.eml-file) (what makes perfectly sense for signed emails)

excerpt from the received email's source code:

--------------
Content-Type: message/rfc822;
 name="test.eml"
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment;
 filename="test.eml"

X-Mozilla-Keys:            
To: 

And thereby it leaks the users email agent.