The propagation activity statement discusses the relationship between mail (SMTP), news (NNTP), and web (HTTP) transport. These transport are closely related as they all are message based - that is, they all carry messages around. This is quite different from many distributed systems which transports method invocations. See also:
The MHTML work is relavent to this issue because the static representation of web pages defined here for email can also be used for caches. The same metadata issues arise.