Comment by cryptonector

Emails should really be identified by Message-Id (which isn't guaranteed unique, but is very selective) and a hash of the body and a subset of the headers (e.g., excluding Received headers, and maybe using only Message-Id, Date, and From, maybe not even Subject).

A good email store is very searchable, and a good MUA searches email, and a good MAP gives the client temporary (ephemeral) handles for "open" emails.