Apache James 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.
License: Apache License 2.0
More information: http://james.apache.org/mime4j/