Three delivery events fix the change in time. On June 23 the county address generated an authenticated Microsoft automatic reply linked to the outgoing message. On July 14 the same address returned RecipientNotFound. On July 29 the county edge returned Access denied. The raw RFC 822 hashes, message linkage, rejection hosts, and timing are preserved in a seven-page metadata packet.
What changed
The record establishes a change in recipient resolution or recipient-object status after June 23. The two later codes differ, but both record nonacceptance of the same address.
What did not become knowable
These delivery events do not identify the administrator, policy, separation event, or lifecycle action that caused the change. They do not prove the mailbox contents were deleted. They do not prove DeRosa personally did anything to the account.
The records that answer the real question
Microsoft 365 Unified Audit Log entries, Entra ID lifecycle events, Exchange mailbox state, retention policy, litigation-hold status, forwarding rules, eDiscovery preservation, and administrator audit events can show what happened to the object and whether historical content remained preserved.