Some hosts/domains even mirror Mastodon addresses and email addresses. There is nothing stopping an address from being both, especially if you control your own domain.
Your e-mail address isn't tied to a server. It can be, but it doesn't have to be. I've had many e-mail services without changing my e-mail address.
I can use g-suite, fastmail, outlook for business and a range of other things, and the people sending me mails won't notice. I can't swap my Mastodon server.