Signing and encrypting of messages is possible using emacs-mime, most easily accessed through the Attachments-menu while
composing a message, or with M-x mml-secure-message-encrypt-pgp, M-x
mml-secure-message-sign-pgp.
Important note: the messages are encrypted when they are sent: this means
that draft messages are not encrypted. So if you are using e.g.
offlineimap or mbsync to synchronize with some remote IMAP-service, make
sure the drafts folder is not in the set of synchronized folders, for
obvious reasons.