That would be useful for the drab use case of end to end testing. Every team had probably half arsed a test that needs to check an email inbox via smtp.
I used to use https://www.mailslurp.com/ for that purpose and it worked fairly well. IIRC they even had a long-lived Keep-Alive endpoint where you would make a GET request and it would sit and wait for up to a few minutes until a message arrived.