> an HTTP URI
Note that there's no requirement for something to be an HTTP URI specifically. Any URI will do.
Yet still, without a webmail client (a client that specifically takes an offline protocol and puts it on the “web”) email would not have a URI. You an “link” to a mailbox, but not create a particular mail with a particular identifier then later retrieve it with that same URI. The URI identifies the users mailbox, NOT their mail.
Serving public URIs for messages must be done by separate web servers like a webmail client, or a mail archiving tool such as Pipermail.