# ESP

> What an email service provider actually is, which parts of delivery it runs, and how to tell a real ESP from a platform reselling someone else's pipes.

Canonical: https://emailfast.dev/glossary/esp

An ESP (email service provider) is a company that sends email on your behalf: it runs the sending infrastructure, manages authentication and reputation, and gives you an API or an interface on top. The term hides a real distinction, because some providers operate their own delivery pipes while many others quietly resell someone else's.

## Why it matters

Who actually runs the mail servers determines who can fix your delivery problems. A platform that resells delivery through an upstream provider cannot see connection-level SMTP behavior, cannot control [IP warmup](/glossary/ip-warmup) or IP assignment, and inherits the upstream's policies, pricing, and outages along with its reputation decisions. None of that makes resellers bad products; the interface on top may be excellent. But it is a fact worth knowing before you depend on one, and it rarely appears on the pricing page.

## In practice

every newsletter platform we surveyed rents its delivery: beehiiv's status page lists SendGrid components; Buttondown's subprocessor list names Mailgun and Postmark; Substack sends through Mailgun (verified on their own public pages, July 2026). You can run the same check on any platform yourself: read the component list on its status page, read its subprocessor disclosures, and read the `Received` headers of a message it sent for you. The pipes leave fingerprints in all three, and a platform whose status page lists another ESP's API as a component has answered the question.

## How Email Fast handles it

Email Fast is the other kind: Email Fast runs its own mail transfer agent, warmup engine, reputation breaker, and per-tenant fair queue — the pipes are ours, not resold. The tradeoff is stated plainly: pipes are harder to run than to rent, and live delivery opens at launch rather than being live today, but when delivery breaks, the people you contact are the people who can fix it, at the SMTP connection where it broke.