Philip shows off some of the work he's been doing on Tinymail, a stripped down, PDA oriented mail library based off of camel, what evolution (AKA a memory hungry pig) uses. Anyway, his patches and graphs show what happens when a group of people are dedicated to optimization. I'm pretty sure that you'll be hard pressed to find a mail client/library that uses under 12mb of memory when downloading 30,000 imap headers, much less 8mb! (8-ish, can't read the graph exactly).
Glad to see this stuff happening, hope some of it makes it back to Evolution so I can start using that again instead of always falling back to good 'ol Mutt for performance / convenience reasons.