And of course the general issue mentioned in this thread was fixed long time ago
Unfortunately, it is a bit difficult to identify the problem with the information you’ve provided
In order to diagnose what went wrong in your case, it would be useful to have more data: what are you trying to achieve, how do you send notifications, what is the error you get.
My pleasure.
Would you prefer, do not hesitate send some more details so that fellow forum members have an opportunity to diagnose what’s happening. Using Postman (postman.com) for API testing also helps greatly in identifying the problem.
Or you can submit a support ticket for your problem here: Submit a Support Ticket
In my experience I received 500 when app wasn’t published (I mean no any native build).
As for User ID instead of email - unfortunately that’s not possible at the moment (this is an additional “pain” for my Integromat scenarios - I have to make an extra call to get user email by ID before sending… ).