[Dovecot] Multilayer mail handling

Hi,

I'm wondering what is the correct architecture and configuration to setup a multilayer mail handling? What I have in mind is something more like what we can found in Web platform architectures.

1. First layer does first checks: user authentication and SSL offloading. If all checks are OK request is proxied to layer 2.
2. Second layer, will actually handle the IMAP/POP3 connection proxied by layer 1, no major checks should be done as already made by layer 1.
3. Finally, layer 3 to do some virus checks on the emails and maybe some other things I can't think about.

Do you have some pointers on how to set up such a mail stack where each layer can be on multiple other hosts?

Best regards, Jimmy