We have a customer using SBS 2003 server (exchange) and Mac clients. We have had the very common problem of Entourage giving the spinning beachball etc when synchronising.
We felt, after some testing, it was basically down to the size or number of items in the folders (they can have 15,000 contacts and 10-20,000 inbox items). We tested in jumps of 1000 items and performance degraded after about 6-7000 items.
My question is what do large companies who are MAC based use for email systems. There must be companies out there who also have large email users - what do they do? surely the answer is not to just have smaller mailboxes as thats not really an option.