email server roundup

we are looking to migrate from EIMS to something (1800 users) that can handle the heavy load better. i know there is a mail server round up being done by Joel Rennich (McTroll) are there any sugessions as to what to use. i think i want to keep my investment in entourage (mac) and outlook (pc) groupware would be nice but not necessary. my short list is kerio, communigate pro and postfix. looks like that is also the order in terms of gui, easy of use, etc. are all three just as fast? sid