Context
Bun corpus position 4039, js/third_party/nodemailer/nodemailer.test.ts, is correctly registered as a todo when the upstream Mailgun credentials are absent. The fixture exercises a real authenticated SMTP/TLS send through Nodemailer.
Remaining work
- Provide the native SMTP/TLS transport surface needed by the copied Nodemailer fixture.
- Exercise authentication and message delivery against a deterministic local SMTP fixture where possible.
- Preserve the upstream secret-gated Mailgun path for end-to-end verification without embedding credentials.
- Return structured SMTP failures with stable code, operation/phase, sanitized endpoint, cause, and useful stack frames; never expose usernames, passwords, recipient addresses, or message contents.
Acceptance
- The local deterministic transport contract is covered permanently.
- The upstream secret-gated test runs when all three SMTP variables are configured and remains an intentional todo otherwise.
- Authentication, TLS negotiation, send, and cleanup errors retain causes and redact sensitive values.
- #66 references the completed slice and its corpus counts.