But if they use POP3, the users would download the messages to their
machines. Surely you don't have 40,000 users with 100MB mailboxes stored
on the mailserver?
No, luckily not. But we have a lot of users with 10-20 MB, quite a lot with
50-100, and some hundred with 100+ MB. Many of our users leave their mail on
the server - and we cannot (and will not) avoid it.
Tell those users to use IMAP then. You know, you can use both IMAP and POP3
- one does not necessary preclude the other from being used. This is what
IMAP was designed for - to access mail on another server. POP3 was never
designed for that, it was only designed to be a quick and dirty hack to
download mail from another mailbox. The ability to leave mail on the server
is just an often-abused side effect of POP3's mechanism that prevents mail
from being lost due to an interrupted download.