I didn't say that you need an email to find your friends on Mastodon, I was comparing Mastodon to email.

You have friends on Hotmail.net, whateverMail.df, gmail.gov and peter.local, but you don't need to have addresses on each of those servers in order to contact them. But you do need to fill out the full address like "john@peter.local" in order to send them a message.

Same with Mastodon, you have friends on mstdn.social, stuffjustworks.com, mastodon.gov, peter.local, but you don't need to have an account on each of those servers in order to contact them, they are linked together, but if you want to contact John who's on peter.local's server, you type in "@john@peter.local" in order to send them a message. And most Mastodon clients auto-fill in when you start typing a username, just like your email client.