Ask Slashdot: Building a Large Email Service

Rewd asks: "I'm looking at implementing a large scale email server (cluster) to handle POP3 and IMAP4 for about 25000 people, including a lot of attachments. I'd like to go for an Open Source solution, but a lot of people around here want to go for Microsoft Exchange on NT. Has anyone here successfully built anything like this? Can you recommend any combinations and components which are particularly efficent, capable, secure and reliable?"