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.
Replying is always legal (copyright-wise, at least, notwithstanding other laws that might prohibit it for some other reason, e.g. CANSPAM), so long as your reply does not include my original message or a portion thereof (aside from parts that I didn't really author in a meaningful way, e.g. email headers).
Replying with quotes, forwarding, or sending a paraphrased/summarized equivalent to my email may or may not be automatically legal depending on your jurisdiction's laws around fair use. Unless you know that it falls under fair use, it's almost certainly safer to treat it as illegal without prior authorization.