I used to use https://www.mailslurp.com/ https://www.mailslurp.com/ for that purpose and it worked fairly well. IIRC they even had a long-lived Keep-Alive endpo

I used to use https://www.mailslurp.com/ 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.