It does, unless I misunderstand what you mean. The original URL RFC, RFC 1738 (http://tools.ietf.org/html/rfc1738 http://tools.ietf.org/html/rfc1738), envision

It does, unless I misunderstand what you mean. The original URL RFC, RFC 1738 (http://tools.ietf.org/html/rfc1738 http://tools.ietf.org/html/rfc1738), envisioned three URI schemes relating to email. The mid: scheme would refer to messages by message-id, the cid: scheme to parts of a MIME-encoded message by content-id, and the mailserver: scheme would refer to resources on a mailserver. The mid/cid schemes were further specified in RFC 2111 (http://tools.ietf.org/html/rfc2111 http://tools.ietf.org/html/rfc2111) a 1997 proposed standard, updated in 1998 (http://tools.ietf.org/html/rfc2392 http://tools.ietf.org/html/rfc2392). These do what you want, I think: mid:message-id is a URI referring to an individual message. The mailserver: scheme was never further defined, and was formally moved to the history books in 2011 (http://tools.ietf.org/html/rfc6196 http://tools.ietf.org/html/rfc6196).