> I want to recommend you IMP. You can set IMAP server on firewall and
> IMP/Horde.
I've been playing with SilkyMail (http://www.cyrusoft.com/silkymail),
which is an enhanced version of IMP. It's under the GPL, but Cyrusoft
are offering commercial support packages for it. I'm very impressed,
and (under the right circumstances, which mine weren't) it's a snap to
build as all the tools (apache, php, etc.) are included in the tarball
and a single make command builds the whole environment. Then you untar
the php files into the htdocs directory, run a config script, and
you're in business.