Re: Up, down?
No you can't run a mail server that does anything to a raspberry pi. Don't get me wrong, I love mine, but while in theory you could substitute trucks by cargo bikes.. it won't work.
A decent mail server, with 3000 users, and 20 mails (in/out) per day, that is 60.000 mails per day.
At 40K (work related) that is arround 2,5 GB.
Now sync that with clients, and you get 5 GB , plus protocols, etc.
Probably, more arround 10 GB.
As each of your 2.000 users will have at least 350 MB (and you are really cheap if you do that), then you need about 1 GB of storage.
And that is being cheap and a small server.
So no, you can't do that with a raspberry.
As for Exchange being THAT inefficient.. well, no.
Maybe you could put some barebones system, pop3/smtp style.. but that is something different.. just
install exchange server 2010 and see the difference in HDD requirements..
Your server, of course, will have high activity during business hours, not during the night, but almost the same will happen to the "cloud" server, as you have to serve people from a relatively near place.. you might get maybe 20% more "high use hours", nothing more.
Of course, if you live in the cloud, you have to transmit all that data.. and last time I checked 1 Gb ethernet was really cheap, and 10 Gb for the server, had an acceptable price. Now, 10 Gb internet connections are incredibly expensive.