EmailInMailboxIdentifier

Identity for a conversation in a mailbox: thread + mailbox, not email + mailbox. threadid is always non-null, and it's what bulkdeleteemailfrommailbox / markemailsasread / unread already operate on internally - identifying by a specific emailid was only ever an indirection through Email.threadid, and an unstable one at that (a thread's newest message changes as mail arrives).