Get part of a message. How can I use the optional POP3 command TOP to get the beginning of a message?
You can't. At least, with the Sun POP3 provider, you can't.
In JavaMail 1.2, the Sun POP3 provider only uses the TOP command to get the headers of the Message. Any content that is also returned with that usage of the command is ignored. So there's no really good way for an application using JavaMail to get the beginning of a message from TOP.