One of the concerns with email that article raised was: the recipient can click on "reply" -- without encrypting the reply message -- which would send the response together with the original message "quoted" - all of that unencrypted. This is apparently common operational mistake.

Note that this has nothing to do with key exchange, ie putting it on sd card or whatever, would not change anything about the issue raised above.

There is also about a dozen other issues that the article lists, worth reading for sure.