[Dovecot] IMAP proxy configuration

[ben at electricembers.net]

> No because I don't have local domains, I use proxy on public network to allow 
> access to the real servers on the private LAN, but I think you have to use 
> proxy_maybe and 127.0.0.1 for local domains:
>
> http://wiki.dovecot.org/PasswordDatabase/ExtraFields/Proxy

We use the external IP of the local host (or the second dovecot backend) 
because things aren't listening on that interface, FWIW.