mime4j provides a parser, MimeStreamParser, for e-mail message streams in plain RFC822 and MIME format. The parser uses a callback mechanism to report parsing events such as the start of an entity header, the start of a body, etc. If you are familiar with the SAX XML parser interface you should have no problem getting started with mime4j. The parser only deals with the structure of the message stream and won't do any decoding of base64 or quoted-printable encoded header fields and bodies. This is intentional - the parser should only provide the most basic functionality needed to build more complex parsers.
jSieve is a Java implementation of the Sieve mail filtering language defined by RFC 3028. Sieve is an extensible mail filtering language. Its limited expressiveness (no loops or variables and no tests with side effects) allows user created scripts to be run safely on email servers. Sieve is targeted at the final delivery phase (where an incoming email is transferred to a user's mailbox). jSieve is implemented as a language processor that can be plugged into any Internet mail application to add Sieve support.