I'm also into this. We currently have in my organization a BorderWare Firewall server, which gives good security, and basic email capabilities (smtp and pop3), but no global address book and no imap either.
I was thinking of seting up a debian box behind it, inside the network protected by the borderware firewall server. Is for an organization of about 150+ members and email is already configured and working, so is critical that I don't break what I already have.
My idea is to have the debian box fetch emails through pop3 from the borderware, and also allow users to authenticate to IMAP but against the borderware user accounts (something like delegating of forwarding the authentication request to the borderware server, I don't know if this is possible).
Of course, I don't know much about setting up mail servers, so any idea or variation from this setup that I have in mind, will be welcome.
Any ideas on how to configure and mantain a global address book? (via LDAP I suppose)