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.
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 ), Gnus gives the attachment the name “signature.asc” immediately before sending:
(defadvice mml2015-sign (after mml2015-sign-rename (cont) act) (save-excursion (search-backward "Content-Type: application/pgp-signature") (goto-char (point-at-eol)) (insert "; name=\"signature.asc\"; description=\"Digital signature\"")))