You consider email to be without intermediary while social media is with one? Both have some sort of intermediary usually, but in the case of Mastodon (or ActivityPub in general) it at least uses a protocol, just like email.
With email you get to choose your provider (or be your own provider) and in general providers are prety hands off. SPAM filters are a thing but you won't have your email account banned because of a single conversation. That makes email providers act more as facilitators than intermediaries.