Email Service Provider (ESP)

An Email Service Provider (ESP) is a third-party company that offers email hosting and management services, allowing users to send, receive, and manage emails. ESPs typically provide services by managing email servers and other related systems.

The term is used loosely, and its meaning depends on who you ask — see ESP, mailbox provider or MTA? below, because the distinction matters as soon as you start diagnosing a delivery problem.

Who can be an email service provider?

An Email Service Provider (ESP) can be any person, company or organization that provides email hosting and management services. This can include:

• Single person running an email server for him/her or a group of persons like family.
• An organisation or company running their own email system.
• Web hosting companies that offer email hosting as part of their hosting plans.
• Email marketing companies that specialize in email marketing campaigns and provide features such as email creation, list management, and campaign tracking.
• Internet service providers (ISPs) that provide email accounts to their customers.
• Cloud-based email providers that offer email services through a web-based interface or application.
• Enterprise-level email providers that offer advanced email management and security features for large organizations.

In general, any person, company or organization that has the infrastructure and expertise to provide reliable and secure email services can be an Email Service Provider (ESP).

ESP, mailbox provider or MTA?

In the deliverability world, “ESP” is often used in a narrower sense than the broad definition above, and three terms get mixed up regularly:

• ESP (sending platform) — a service you send through. It provides an API or SMTP endpoint, manages the sending IP addresses and their reputation, and handles bounces, unsubscribes and campaign reporting. Examples: Mailchimp, SendGrid, Postmark, Brevo, Amazon SES.
• Mailbox provider — a service that hosts the recipient’s mailbox and decides whether your message reaches the inbox, the spam folder, or nothing at all. Examples: Gmail, Outlook.com, GMX, Web.de, Yahoo. These are the systems whose filters you are actually up against.
• MTA — the software doing the actual SMTP transfer, such as Postfix or Exim. An ESP runs MTAs; so does a mailbox provider; so does a company running its own mail server.

The practical reason to keep them apart: if mail is not arriving, “the ESP” and “the mailbox provider” are two different parties with two different sets of logs, and the fix lives with one of them. An ESP can tell you the message was accepted; only the mailbox provider decides where it landed.

What does an ESP do for deliverability?

A sending platform typically takes on work that is otherwise yours to do:

• Maintaining a pool of sending IP addresses with established reputation, and keeping them off IP blacklists.
• Setting correct reverse DNS for those IP addresses.
• Publishing and managing SPF, DKIM and DMARC records, or giving you the records to publish on your own domain.
• Processing bounce messages and suppressing addresses that hard bounce, so you do not keep sending to dead mailboxes.
• Handling unsubscribe headers and complaint feedback loops from mailbox providers.

None of this removes your own responsibility for what you send. Reputation follows the sending domain as well as the IP address, and a shared IP pool means your results are partly affected by the other customers on it.