LARGE scale email

[Phoenician]

<snip>
1) I am migrating email from a Win32 environment to a *nix based
environment. :) I am working with people that I have to remind that
Linux is not a singular all-encompassing distro (ie: that Linux does
means Linux but rather SuSe, Redhat, Mandrake etc..)
2) It must be ultra-secure.
3) Large scale means over 1,500 mail boxes and approx 350 to 400
domains.
<snip>

Thank you to everyone that took the time to reply.  I have a lot of
reading ahead of me but I can at least say that so far I have chosen to
go with OpenBSD.  I have installed the OS on a test computer and I'm
tinkering as much as possible right now.  - Michienne