This all depends on how the administrator of the Exchange server has it configured. The short version is that if POP or IMAP is allowed then just about any email client for the N810 will work for receiving email. If SMTP is allowed then you can also send using most any client. If Outlook Web Access is allowed you cab use a browser for sending and receiving email.

If only native Exchange protocols are allowed then you are out of luck, as far as I know, as there is no client for Maemo that supports that protocol.