I suggest designing APIs so that they can return warnings and still return useful data, as opposed to only success with data, or no errors. (https://docs.influxdata.com/influxdb/cloud/api/#tag/Response...)

Out-of-band data, like emails, is bound to be ignored by some.

I'd incorporate warnings in the application-specific responses; you can also return a different response code to make sure many clients do not blindly ignore warnings. The HTTP standard includes a 207 Multi-Status response code, which is mostly used with WebDAV.