(2013-08-19)
Make sure IMAP is enabled for your mailbox on the Exchange server.
The relevant configuration items for mutt are:
set spoolfile=imaps://mail.mygridway.net:993/INBOX
set folder=imaps://mail.mygridway.net:993/
set imap_user="<username>@<mymaildomain>"
set postponed="imaps://mail.mygridway.net:993/Drafts"
set record="imaps://mail.mygridway.net/Sent Items"
imap_user is the login name you would present to Exchange in other circumstances, ie through OWA; for example:
dave.mackintosh@xdroop.com