> Does anyone have any good resources on how to get better at doing "functional core imperative shell" style design?

Hexagonal architecture[0] is a good place to start. The domain model core can be defined with functional concepts while also defining abstract contracts ( abstractly "ports", concretely interface/trait types) implemented in "adapters" (usually technology specific, such as HTTP and/or SMTP in your example).

0 - https://en.wikipedia.org/wiki/Hexagonal_architecture_(softwa...