LARGE scale email

[Phoenician]

  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.