LARGE scale email

Just out of curiosity, what would people recommend for large scale email? (what distro and application) I'm not wanting to start a flame war, just get opinions from people in that have had more experience than I have in large scale email. That said, here are the factors for consideration:

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.

Thanks,

Michienne.