I often see some comments and gibberish about aoviding the spyware of the IM's fucksapps.

One of these have the ideas of setting up a mail server with sendmail, dovecot or postfix.
Bur, for a small user, let's say your friends troupé, aside of the dedicated machine, isn't needed a configuration, account or service from an isp to enable the ip's to be reached for?

Otherwise, I would have only a local server or at least need some NAT to enable it to be reached outside the LAN.
Is this rigjt?
or
how can enable my local mail server to have internet in and out data?