[mmbop]

They should have a debug mode that is user-activated for stuff like this.

I also have burned too many hours trying to get various OAuth flows working.

[magicalhippo]

Yeah some development servers that didn't actually send the mail anywhere but did give usable error messages for example would be amazing.

To avoid it being used as an attack vector they could be tied to special app registrations that had to be registered with the mail development system in advance.