Oh! that was a user error, I thought for a moment about the prefix but didn't pay enough attention.

Since, the customers for a service like this would likely be bulk emailing services and those who host their own mailing lists with thousands of users I think encapsulating the entire sending/receiving email part of this and offering a simple API would be the way to go.

As there's no barrier to entry from technical perspective, the size of email id pools(from different email service provider), ability to detect outage accurately and in realtime would be the only differentiator; So encapsulation of those can help monetisation.