Hunnysoft sample mime messages (local copy)
Real world MIME messages generated by older MUAs (Outlook 00, Netscape Communicator 4.7, Eudora 4.2, PINE).
PHP mimeparser test suite
Seven messages, testing RFC 822 and mime parsing, including RFC 2047 internationalization
MimeKit unit test data
Include S/MIME, TNEF and message/partial tests, but not much vanilla MIME