We have a local wireless network (>3000 mailboxes spread over 20 domains).
However, as we're situated in a rural area, we do not have access yet to >4Mbps ADSLs.
I've been running 3 Zimbra servers for some years now, each on a dedicated DSL line and apart from occasional sending backlogs, it's been working great.
The advantage of the server being local, is that sending/receiving from the user perspective is very fast and mail can trickle in/out at it's leisure.
Our problem however is that we also sell DSL accounts and we have users that travel, so they need to receive mail (POP/IMAP/ZWC/ZD)from outside the network, i.o.w. over the same oversaturated DSL connection, which varies from slower-that-dial-up to non-exsistent, which is to be expected.
We can have a remote server directly on Internet backbone, which would be great for remote users, but slower for local users.