One of the servers I operate has about 2,000 mailboxes where nearly everyone uses IMAP, the maximum message size size (with all attachments) is 30MB. We regularly have people e-mailing large files (CAD drawings, PDFs, other images, etc.) around as part of their regular business. We get no complaints on speed or downtime. This all runs on a Dell PowerEdge 2950 with plenty of speed, storage, and bandwidth. Snapshot of the settings and mailbox store taken every three hours in case the server fails. I'll leave out our operating system and e-mail server software to avoid a flame war, but it works really well. We're not counting years between reboots on that system though (months and it's usually due to an OS update or e-mail server software upgrade).