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Do not build this. Deliverability and warmed sending addresses are the product, and reputation cannot be bootstrapped.

Application mail that does not arrive is an outage you cannot see from your own side. A relay gives you infrastructure with an existing standing at the large providers, plus the bounce and complaint feedback that keeps it.

WHAT TO DO INSTEAD OF BUILDING
- Rent a relay. Compare on price at your volume and on the quality of the feedback you get back
- Never run your own mail server for application email. Clean address, reverse DNS, warmed reputation and feedback loops with every provider is a job

WHAT YOU MUST DO REGARDLESS
- **SPF, DKIM and DMARC**, with DMARC enforcing rather than sitting at none, and the aggregate reports actually read
- **A separate subdomain for transactional and marketing mail.** A password reset failing because a campaign got complaints is the specific disaster to avoid, and sharing a domain is how it happens
- Warm a new domain slowly: hundreds, doubling over weeks
- Bounces and complaints consumed and acted on: hard bounce suppresses immediately, complaint suppresses permanently and is never mailed again
- A per-recipient daily cap, so a loop cannot send somebody four hundred messages
- One delivery row per recipient per message, written before sending, idempotent, with the provider's message id recorded. That makes a resume exact and lets you change provider by rewriting one file

WHAT TO BUILD
- **A local mail catcher for development.** Nothing reaches a real person by accident, nobody needs a provider account to work, and you can read the raw source, the HTML and the headers of what your application actually sends. That is under Mailtrap elsewhere in this catalogue and it is a weekend
- Templates as MJML or tables with inline styles, with the plain-text alternative generated from the same source rather than written twice. Preview in the client that renders with the Word engine, because that is the constraint

THE CHECK NOBODY RUNS
Send your own password reset to three different large providers and see where each lands. Most people have never looked, and the answer is occasionally the spam folder — which explains a support queue nobody could account for.

THE ONE-LINE VERSION
Rent the relay, keep transactional apart from marketing, build the development catcher, and go and check where your reset email lands.