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Apache James Mime4j
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"Apache 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. "

For more information:
http://james.apache.org/mime4j/