It returned Promise<void> and returned early in two cases that were indistinguishable from success at the call site: no SMTP credentials, and a recipient outside MAIL_ALLOWLIST. A caller could therefore report that it had emailed somebody a message nobody would ever receive, and in QA — which restricts delivery deliberately, as its entire safety property — that is the normal case rather than an edge one.
It now returns a MailOutcome saying which of the three happened. No existing caller changes: there are seven and every one ignores the result, so this is additive. Re-deriving the answer at a second site would have duplicated isAllowedRecipient and the SMTP check, which is exactly the drift the guard-in-one-place comment above them exists to prevent.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
