Much of my day was spent working on a bug described in this SO question.
In a narrow sense, it was caused by a careless error: it was a hanging " in one of my environment variables.
But in the broader sense, the problem wasn’t the typo. It is my unfamiliarity with Next.js, which made me look and double-check all these areas:-
- client/server components
- different .env file settings for each
- the concept of components itself
- Next directory structure
- mailgun create method
Had I known the eco-system better, I wouldn’t have looked so widely: the typo would have been spotted sooner.