Over in the enigmail mailing list:
https://admin.hostpoint.ch/pipermail/enigmail-users_enigmail.net/2017-February/004323.html
Patrick Brunschwig writes about converting an encrypted message into a cleartext
message:
> If the message was signed and encrypted in one step, then the signature
> will be removed as well. GnuPG does not have a functionality to extract
> the signature from the encryption (which would theoretically be
> possible).
If this functionality was provided, then enigmail could presumably preserve
signatures on the transformed message.
I don't personally think this is a high priority, since i think a better
approach (over storing the message as cleartext) is to simply cache the session
key to facilitate future decryption without secret key access.
But i wanted to record the observation of the missing feature here.