SendGrid

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Do not build a mail server. Build the sending layer around a relay so you can change relay in an afternoon.

STACK
- Node with Fastify, SQLite for the outbox
- A relay behind an interface: Postmark, SES and Resend adapters, chosen by an environment variable

THE OUTBOX
- Every message written to the database before any send is attempted
- A worker claims rows with a lease, so two workers cannot send the same message twice
- Idempotency key per message; a retry after a timeout never sends a duplicate

TEMPLATES
- MJML or a small component set compiled to table-based HTML, with a plain-text alternative always generated
- Rendered and stored with the message, so what was sent is reconstructible years later

EVENTS
- One webhook endpoint per provider, normalised into one internal event shape
- Bounces and complaints write to a suppression table checked before every send, forever
- Verify webhook signatures; an unsigned event is discarded and logged

RATE AND VOLUME
- Global send rate honouring the relay's limit, with a queue that survives a restart
- Alert when the complaint rate over the last thousand sends passes 0.1 per cent

WHAT YOU ARE RENTING AND WHY
- IP reputation. The README should say plainly that self-hosting Postfix means being blocked by Microsoft for months

Include a README on SPF, DKIM, DMARC, warming, and the cost per hundred thousand emails on each adapter.