Email is one of the few parts of any system that always needs to use a queue, because of the need to be asynchronous and to occasionally delay/retry. It's not

Email is one of the few parts of any system that always needs to use a queue, because of the need to be asynchronous and to occasionally delay/retry. It's not a great example. Your distinction might make sense to you, but it's technically wrong. An API means a specific thing. You're almost saying that a car door and a house door aren't both doors. A library has an API and a directly called microservice has an API and a message based microservice has an API. A 'contract', as you call it, is an API.