See https://grepular.com/Automatically_Encrypting_all_Incoming_Email

This application takes one argument on the command line. The email address to
look up the public key that the message will be encrypted with. An email
message is piped through the application, and the resulting email is sent to
STDOUT encrypted with the relevant public key. If you provide multiple email
addresses, then the message will be encrypted with multiple keys.

If the message is already encrypted, it doesn't get encrypted a second time. If
the message has multiple parts, PGP/MIME is used, otherwise inline PGP is used.

Exim users can use the transport_filter directive in a transport in order to
call this application, like so:

  transport_filter = /bin/gpgit.pl my.email.address@example.com

Procmail users can add a procmail recipe as follows

  :0 f
  | /bin/gpgit.pl my.email.address@example.com

If you call gpgit.pl from a different application, I'd love to hear from you so
I can update this README file.