exJello is a JavaMail provider that connects to a Microsoft Exchange server using the WebDAV interface exposed by Outlook Web Access.  It is designed as a drop-in replacement for the standard JavaMail POP3 and SMTP providers.  This allows you to send and receive messages through your Exchange server in situations where a POP3/SMTP interface is not available (through a restrictive firewall, for example, or if your administrator simply does not provide a POP3 or SMTP gateway).