exJello is a JavaMail provider that connects to a Microsoft Exchange server (actually, it uses the WebDAV interface exposed by Outlook Web Access). It is designed as a drop-in replacement for the standard 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).
Simple Java Mail aims to be the simplest to use lightweight mailing framework for Java, while being able to send complex email messages including attachments and embedded images. The Simple Java Mail framework essentially is a wrapper around the JavaMail SMTP mailing API that allows users to define email messages on a high abstraction level without having to deal with terminology such as "multipart" and "mimemessage".