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Do not build this. IP reputation with mailbox providers is the product, and it takes years to accumulate.

Running your own mail server for bulk sending means a clean address, reverse DNS, warmed reputation, feedback loops with each provider and constant attention. Do it badly and your mail silently stops arriving — an outage invisible from your own side.

WHAT TO DO INSTEAD
- Rent a relay, and compare properly: providers differ by a lot at the same volume, and by more in the quality of the bounce and complaint feedback
- Consider a dedicated address only above real volume. Below it, a shared pool with a good provider is better than a cold dedicated one, because a dedicated address with low volume never builds a reputation at all

WHAT YOU MUST DO REGARDLESS
- **SPF, DKIM and DMARC**, with DMARC enforcing rather than sitting at none, and the aggregate reports read. They tell you who is sending as you
- **Separate subdomains for transactional and marketing.** A password reset failing because a campaign got complaints is the disaster this prevents
- Warm slowly: hundreds, doubling over weeks. A new domain sending twenty thousand on day one stays in spam
- Bounces and complaints consumed and acted on: hard bounce suppresses immediately, complaint suppresses permanently and is never mailed again
- One-click unsubscribe and the List-Unsubscribe headers on anything bulk, which the large providers now effectively require
- Watch the complaint rate daily. Above a fraction of a percent an account is suspended, and that is the failure that ends everything
- Suppress anyone who has not opened anything in a very long time. A list of dead addresses is what destroys a reputation

WHAT TO BUILD
- 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
- A per-recipient daily cap, so a loop cannot send four hundred messages
- A local mail catcher for development, so nothing reaches a real person by accident — under Mailtrap elsewhere in this catalogue, and a weekend

THE ONE-LINE VERSION
Rent the reputation. Keep transactional apart from marketing, and build a sending layer thin enough to swap providers in an afternoon.