IMAP Gateway: signed mails

new feature

PGP-signed mails should be enough.

(smime/pgp/...)

Mapi itself does not seem to support specific features for the transport or storage of encrypted emails.

pgp-signing should work already, as it is controlled by key phrases in the content body. When using Thunderbird to store and read a pgp-signed mail that works fine. Evolution can read the mail as well.
Problems show, when using Outook via MAPI to read the mail.
If Outlook stores the mail via MAPI all parties can read it.

S/Mime:
the content type multipart/signed must be created when rebuilding the messages. That can probably done by checking the content type of the second attachment and then reset the subtype of the multipart to signed.

Questions:
* should we store encrypted emails
* should we encrypt/decrypt them only for transport
* does MAPI provide any support/definitions around this topic

RFC1847 (might be outdated):
  Content-Type: multipart/signed; protocol="TYPE/STYPE";
            micalg="MICALG"; boundary="Signed Boundary"

    --Signed Boundary
    Content-Type: text/plain; charset="us-ascii"

    This is some text to be signed although it could be
    any type of data, labeled accordingly, of course.

    --Signed Boundary
    Content-Type: TYPE/STYPE

    CONTROL INFORMATION for protocol "TYPE/STYPE" would be here

    --Signed Boundary--