I would like to know if it is possible to communicate with people that are not logged in to the forum through the email interface?
Yes - when reply by email is enabled, users do not have to visit the forum to receive categories/topics to which they are subscribed (or following). They will also receive private messages.
Even if the sender has no account on the forum?
No, they must have an account.
Edit: I stand corrected. ↓
Actually it is possible. Search the site for “Staged Users”.
More info here: Start a New Topic via Email ✉
This is a very good news! Thanks for the information!