[Dovecot] Authentication issue

Hi,

I need to set up a weird dovecot configuration:

1. outside a ssl tunnel I'd like to authenticate only with cram-md5 scheme
2. inside a ssl tunnel I'd like to authenticate only with plain auth

The first is easily satisfied with

auth_mechanisms = plain cram-md5 disable_plaintext_auth = yes

but I don't know how to satisfy the second condition, if it's possible.

Thanks for help, Emiliano Rago