Emacs: Crypto: How to get automatic encryption?

[Gregor Zattler]

Dear notmuch developers,

I replied to a PGP/MIME signed and encrypted e-mail and was
astonished to realise that my reply was sent unencrypted (and
unsigened for that matter).

Actually the message buffer in which I write replies has Auto
Encryption Mode enabled, but nothing happens.  

Till now I investigated customisable variables for notmuch,
message mode, epg but did not find a way to tell Emacs to sign
and encrypt an e-mail if the correspondent (signed and) and
encrypted her/his e-mail.  

Is this doable?   How so?

Thanks for an answer, happy new year and thanks again for notmuch, Gregor