This depends. If you are in a corporate environment and your personal files
are on a server, then the IT people may be able to get it back for you. If
you have "Leave a copy of messages on the server" set and "Remove from
server when removed from 'Deleted Items'" not set (Advanced Tab), then you
can download another copy from your e-mail server. It may possibly be in
the Recycle Bin. There are a few places to check, but it is probably gone.
Sometimes if you ask nicely the sender has it in "Sent Items" and can send
you another copy.