ESP vs ISP: who controls what in email delivery

ESP and ISP are frequently used loosely and sometimes confused in email discussions, but they sit on opposite ends of the delivery path and neither controls the other.

ESP: the sending side

An Email Service Provider is the platform you use to send mail - SendGrid, Mailgun, Postmark, Amazon SES, or a marketing platform like Mailchimp or Klaviyo. The ESP controls:

• The sending infrastructure (IPs, in most cases shared with other customers unless you've paid for a dedicated IP)
• Bounce handling and suppression-list mechanics on their platform
• Whatever authentication support they offer (custom DKIM, custom MAIL FROM for SPF/DMARC alignment)
• Rate limits and sending policies they enforce on your account

An ESP cannot guarantee inbox placement. It controls everything up to the point mail leaves its servers; what happens after that is entirely the receiving side's decision.

ISP: the receiving side (more precisely, the mailbox provider)

The term ISP (Internet Service Provider) is used loosely in email contexts to mean the mailbox provider receiving the mail - Gmail, Outlook/Microsoft, Yahoo. The more precise modern term is mailbox provider (MBP), since Gmail and Outlook aren't providing your internet connection, they're providing (and filtering) your mailbox. Whichever term is used, this side controls:

• The spam filter and inbox-placement decision for every message
• Sender reputation tracking, specific to their own system (Gmail's reputation signals are separate from Microsoft's)
• Rate limiting and throttling of inbound mail from a given sending IP or domain
• Feedback mechanisms like Google Postmaster Tools or Microsoft SNDS/JMRP, which expose some reputation data back to senders

Why the distinction matters practically

If mail isn't arriving, the fix lives on different sides depending on the cause:

What not to do

• Don't contact your ESP's support expecting them to fix a Gmail-specific filtering decision - they can confirm the message left their servers correctly, but they don't control Gmail's filter.
• Don't assume a good relationship or history with one mailbox provider transfers to another - Gmail, Outlook and Yahoo each maintain independent reputation views of your sending domain.
• Don't treat "the ESP says it delivered" as the end of the investigation - delivered to the receiving server and landed in the inbox are different claims, and most ESPs only report on the former.

A worked example of where troubleshooting goes wrong

Say a marketing team notices open rates dropping for mail sent to Outlook addresses specifically, while Gmail and Yahoo opens stay flat. The instinct is often to contact the ESP, since that's the platform the team has a support relationship with. But the ESP's logs will show the message left their servers successfully in all three cases - there's nothing on their side to investigate, because the problem is entirely on Microsoft's side: a reputation or filtering decision specific to how Outlook views this sending domain. The right next step is checking Microsoft's own SNDS (Smart Network Data Services) or JMRP (Junk Mail Reporting Program) feedback for that sending IP or domain, not escalating further with the ESP.

Why some people use "ISP" loosely and what's technically correct

Historically, your actual internet service provider (a cable or telecom company) often also provided your email address, which is where the habit of calling a mailbox provider an "ISP" in email contexts comes from. Today, Gmail, Outlook.com, and Yahoo Mail are mailbox providers, not internet service providers in the literal sense - almost nobody gets their email account bundled with their home internet connection anymore. The term persists in casual usage, but "mailbox provider" (MBP) is the more accurate term for the entity actually making inbox-placement decisions.

A short glossary, since the terms get mixed up

• ESP (Email Service Provider): the platform you send through.
• ISP (Internet Service Provider): technically your internet connection provider; used loosely in email contexts to mean the receiving mailbox provider.
• MBP (Mailbox Provider): the more precise modern term for Gmail, Outlook, Yahoo and similar - the entity actually hosting the recipient's inbox and deciding placement.
• MTA (Mail Transfer Agent): the server software actually moving mail between systems, which can sit on either the ESP's or the mailbox provider's side.

Using "mailbox provider" instead of "ISP" in any internal documentation or reporting avoids the confusion this mismatch causes, particularly for anyone newer to the space who takes "ISP" at its literal, original meaning.

MailPilot measures the receiving side directly - actual per-provider inbox placement through a real seed-inbox network - rather than relying on what your ESP reports as "delivered," which is the piece neither side alone can tell you.