There are some PGP plugins for outlook which may be able to decrypt the message you send them with PGP/MIME, but encrypted attachments will show up in outlook with a random name and the file extension .dat.

When searching the interweb for help I found lots of rants and just a few hints[1][2], thus I will describe a workaround.


First of all, install MailCrypt.

Then add this elisp code to your ~/.gnus.el.

   ;; "put Content-Type: text/plain at top of empty parts" (gnus manual)
   (setq mml-insert-mime-headers-always t)
   ;; encrypt always with my key
   (setq mc-encrypt-for-me t)
   ;; don't sign the message
   (setq mc-pgp-always-sign 'never)


The problem seems to be that Outlook doesn’t like unnamed attachments. Using the following code (based on settings from Sigma [3]), Gnus gives the attachment the name “signature.asc” immediately before sending:

  (defadvice mml2015-sign (after mml2015-sign-rename (cont) act)
      (search-backward "Content-Type: application/pgp-signature")
      (goto-char (point-at-eol))
      (insert "; name=\"signature.asc\"; description=\"Digital signature\"")))