http://www.ietf.org/rfc/rfc1864.txt
Despite all of the mechanisms provided by MIME [1] which attempt to protect data from being damaged in the course of email transport, it is still desirable to have a mechanism for verifying that the data, once decoded, are intact. For this reason, this memo defines the use of an optional header field, Content-MD5, which may be used as a message integrity check (MIC), to verify that the decoded data are the same data that were initially sent. The Content-MD5 header may also be placed in the encapsulated headers of an object of type message/external-body, to be used to verify that the retreived and decoded data are the same data that were initially referenced.

MD5 is an algorithm for computing a 128 bit "digest" of arbitrary- length data, with a high degree of confidence that any alterations in the data will be reflected in alterations in the digest. The MD5 algorithm itself is defined in [2]. This memo specifies how the algorithm may be used as an integrity check for MIME mail.