Question : Best Emailing System

Hi,
I am currently using Sendmail as my emailing system where i am not confident on high-availabity and the soution is not user freindly and not meeting my requriements. How to indenfity the best emailing solution for corporates where email communication from customer, supplier, internal employees, auto-emails and romaing users.
Is there any thumrule or bench mark to evaluvate the robust cost effective solution

Answer : Best Emailing System

Here's a very recent comparison between Exchange and Domino (i.e. the mail server in Lotus Foundations):
http://www.itcomparison.com/Mail/Exchange2k7vslotus8/Exchange2k7vslotus8.htm

How the others do I don't know.