[Dovecot] proxy and gpg processing

Hi

I have dovecot running as pure proxy for accessing imap and pop mailboxes on other servers.

How can I do the following scenario:

When a client user reads a gpg signed or gpg encrypted email through the proxy, can I verify/decrypt the email before it gets delivered to the client user?

Ideally I would like to write a perl program which processes the email within the proxy before it gets delivered to the user. With Sieve I can not do this, is there another way to achieve this?

Thanks for any hint. Mike