There is less of defederation in the email network, because mail has been build to reach its destination no matter the path or the time, message must arrives even partially. (this was a premise of the US military, at ARPAnet). Even if a mailadmin blocks one server, mail could go another route. This is also the base of the internet, path is not the most straight forward direct link between source and destination (companies are usually against this structure)

AT or Mastodon don't have this freedom or constraint (depend of one point of view).