[Dovecot] Auto-responder to handle unencrypted (and/or unsigned) email messages whose origin is not localhost

[Ben Johnson]

Hi, everyone,

I'm surprised how little exists on the Internet regarding this
particular subject. Given the recent headlines, that is about to change,
I'm sure.

I have a need to send an automated response to all senders who attempt
to send unencrypted and/or unsigned email to a specific mailbox on my
system. (Messages originating from localhost should be exempt.)

I would like for the auto-reply to include a customized message with the
appropriate public PGP key contained therein.

Has anyone accomplished this? If not, some basic theory as to how one
might go about achieving this would be greatly appreciated.

Perhaps this is a job for Dovecot's "sieve_before" directive. Would this
simply be a matter of checking the message body for the expected
patterns, e.g., starts with "----- BEGIN PGP MESSAGE -----", etc., etc.
and sending an automated reply with the public key if the expected
strings are not present?

Any additional thoughts would be very helpful!

Thank you in advance,

-Ben