TL;DR: For a Node.js service that sends a welcome email with a generated logistics report, choose the API that makes the message traceable: a reusable template, a transactional send, suppression checks, and records that connect the report to the request. A single REST surface can reduce credential and adapter sprawl, but it does not remove the need for domain policy, retention rules, or an application-owned evidence store.

The first trade-off is concentration versus operational friction. Separate mail providers can give an organization more vendor choice; a unified backend surface can give the worker one key, one bill, and one HTTP convention across services. For compliance, the winning design is the one that makes the chain of evidence easy to reconstruct.

Keep it bounded.

What evidence must a welcome email leave behind?

Treat the email as an event in the report workflow, not as the workflow itself.…