Email has two very distinct purposes, not 1, not 3.
1- federated pseudonymous identity protocol for internet(ip/http).
2- medium-length business time insensitive communication.
That's it.
This sounds like a free experimental product aimed at improving 1, but most service providers will either have to block it or face abuse from users of your domain.
What OP doesn't know yet probably is that they will not be the ones under dos/spam,rather they will be used to DoS, spam, simulate traffic, scam, etc...
Edit: the ToS outlaws spam, so it is possible they will cooperate with spam blacklist providers like spamhaus ( not sure how). Not sure how businesses will block the domain for acct creation, probably as a whole, Ive seen websites use a whitelist approach like gmail, or outlook.