I don't think it matters. the specific ways in which email services work or are used are not what the analogy is supposed to explain.
it's supposed to explain how two people who log in to different lemmy instances is different from logging into Facebook and MySpace, or Twitter and Threads.
"how does it work? aren't they different sites?'
"you know how you can have a gmail and someone else can use an outlook email but you can still send emails to each other"
done. even 70 year olds would get it. problem solved. easy, approachable analogy.