(in addendum to what is listed above):
More accurately
Exchange is a
collaboration system, since it
handles more than
mail. It also deals with
calendaring,
notes,
journals, server-side
rules, etc.
Lotus Notes shares similar
features and a
similar internal setup.
Exchange is different from many other email systems in that it has a "
store", or
binary database where email, contacts, and files are stored. The largest theoretical
advantage over a
flat mailer is this: Let's say you have a
company with 3,000
employees, and you want to send them a piece of mail. Then with this setup, for 3,000 employees under
sendmail (no
gripes against sendmail, it's really a nice piece of
software) you have 3,000 copies of this
message. That could be quite taxing for the
server to
replicate. Now with exchange, you have 1 message, and a
reference count of 3000. Much smaller, more
efficient, and more
scalable (in theory). Once someone's
MAPI client removes the message, or deletes it via
IMAP or
POP, then the ref. count is taken down one. A more dramatic example is mailing a huge
movie to 50 people. That would be orders of magnitude more horrible.
Exchange can connect to a variety of
clients including IMAP and POP (not just
Outlook, which does exist for PC and Mac), but you do not get the collaboration features with that type of connection.