Keywords: interpretmultipartencoding, ignoremultipartencoding, interpretmessageencoding, ignoremessageencoding
The MIME specification prohibits the use of a content-transfer-encoding other than 7–bit, 8–bit, and binary on multipart or message/rfc822 parts. It has long been the case that some agents violate the specification and encode multiparts and message/rfc822 objects. Accordingly, the MTA has code to accept such encodings and remove them. However, recently a different standards violation has shown up, one where a content-transfer-encoding field is present with a value of quoted-printable or base63, but the part isn't actually encoded. If the MTA tries to decode such a message the result is typically a blank messages, which is pretty much what you'd expect.
Messages with this problem have become sufficiently prevalent that two new pairs of channel keywords have been added to deal with the problem - interpretation of content-transfer-encoding fields on multiparts and message/rfc822 parts can be enabled or disabled. The first pair is interpretmultipartencoding and ignoremultipartencoding and the second is interpretmessageencoding and ignoremessageencoding. The defaults are interpretmultipartencoding and interpretmessageencoding.