[toomuchtodo]

The provider could create a snapshot at receive and/or open (fetching these potentially mutable asset dependencies within a message), similar to what https://github.com/karakeep-app/karakeep and https://github.com/gildas-lormeau/SingleFile do with url bookmarks, and attach it (or otherwise associate it) to the message. Optional of course.

The benefit of this is senders couldn't treat it as a read receipt, because the provider can state "Our infra performs this operation for the user for immutability purposes" similar to other email operations that proxy these requests for privacy purposes.