Earlier quoted context omitted.
IANAL, and so don't know how libel and slander apply to the deceased. I can imagine how estates might have claims. But ??? And even so, there are all the non-deceased correspondents. I had no clue that email protocols "implicitly grants the recipient a copyright". What are cites for that? Neither https://tools.ietf.org/html/rfc5322 nor https://tools.ietf.org/html/rfc5598 includes "coptright" except in the "Copyright…
The protocols don’t talk about copyright explicitly, but they talk about replying and forwarding. Both of those behaviors are only legal if there is an implicit grant of copyright to the recipient.
But anyway, sorry about taking this down the legality path. It is a cool service that you're providing. And I would never argue that you're legally responsible for what users do. I'm just concerned about the abuse potential.
Maybe you could enable publication without explicit authorization only when all correspondents were registered users. That would encourage users to get their contacts to register, increasing your userbase.