[1] RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types ( 版) http://tools.ietf.org/html/rfc2046#section-5.2.3.5

[2] RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types ( 版) http://tools.ietf.org/html/rfc2046#section-5.2.3.6

Given the potential for

abuse, any such request messages that are constructed

should contain a clear indication that they were

generated automatically (e.g. in a Comments: header

field) in an attempt to resolve a MIME

"message/external-body" reference.