An ESP (Email Service Provider) is the platform that sends, tracks, and routes a brand's emails - companies like Mailchimp, Klaviyo, Iterable, Customer.io, Substack, Beehiiv, ConvertKit, MailerLite, ActiveCampaign, HubSpot, SendGrid, Postmark, and dozens of others.

How does the ESP detector work?

It inspects the public signals every email leaks: SMTP headers (Return-Path, DKIM domain, X-Mailer), the link-wrapper domain (kx.cdn for Klaviyo, list-manage.com for Mailchimp, etc.), the tracking pixel host, and the unsubscribe landing page. Combined, these identify the ESP with high confidence.

Can it detect multiple senders / subdomains?

Yes. Brands often use one ESP for transactional and another for marketing. The detector reports each sender separately and flags brands that recently migrated.

Does it tell me what trackers are embedded?

Yes. Beyond the ESP, the detector lists embedded tracker domains (Customer.io pixels, Litmus tracking, Iterable beacons), the UTM convention used, and the unsubscribe stack. It is the BuiltWith of email.

Yes. The lookup is free and unlimited within reasonable rate limits. Signed-in Newsletrix users also see ESP migration timelines for every tracked competitor.

🔍 Free ESP lookup

Newsletter ESP detector

Find out which email service provider any brand uses. ESP, tracking domain, UTM convention, unsubscribe stack - all detected from public signals in seconds.

Detect an ESP free ✨

30+ ESPs supported Migration history BuiltWith for email

Quick answer

How do you find out which ESP a brand uses?

TL;DR. Every email leaks the ESP that sent it. The link-wrapping host (list-manage.com = Mailchimp, kx.cdn = Klaviyo, email.beehiiv.com = Beehiiv), the SMTP Return-Path, the tracking pixel domain, and the unsubscribe landing page combine into a high-confidence fingerprint. Newsletrix detects 30+ ESPs and reports the rest as "custom / unknown."

Coverage

30+ supported ESPs

Every major sending platform plus the long tail.

Marketing-first

Mailchimp, Klaviyo, ActiveCampaign, ConvertKit, MailerLite, Brevo (Sendinblue), HubSpot Marketing, Omnisend, GetResponse, Drip.

Newsletter platforms

Substack, Beehiiv, Ghost, Buttondown, Revue (legacy), Kit, Loops, Letterdrop, Curated.

Lifecycle / B2B

Iterable, Customer.io, Braze, Marketo, Pardot, Eloqua, Mailgun, Postmark, SendGrid (Twilio), Amazon SES, Mandrill.

Custom / in-house

Detected by elimination - if no fingerprint matches, the detector reports "custom in-house" with the SMTP server domain.

Signals

What the detector reads

Public signals only. We never inspect email body content for personal data.

Signal	Example	What it identifies
Link wrapper	`kx.cdn`	Klaviyo
Link wrapper	`list-manage.com`	Mailchimp
Link wrapper	`email.beehiiv.com`	Beehiiv
Return-Path	`bounce@em.iterable.com`	Iterable
X-Mailer	`Customer.io`	Customer.io
Tracking pixel	`track.litmus.com`	Litmus tracking
Unsubscribe URL	`substack.com/account`	Substack
UTM pattern	`utm_source=newsletter&utm_medium=email`	Klaviyo / Iterable / Customer.io defaults

Who it is for

Use the ESP detector when you need to

Sales prospecting

Build prospect lists by ESP - "every brand on Mailchimp under 50 employees" - and time outreach against migration windows.

Competitive intelligence

Spot when a competitor migrates ESPs - usually a leading indicator of a brand refresh, new CMO, or repositioning.

Tooling decisions

Find which ESP the best newsletters in your niche actually use before you make a switch.

FAQ

Frequently asked questions

What is an ESP?

An Email Service Provider - the platform that sends, tracks, and routes a brand's email (Mailchimp, Klaviyo, Substack, Beehiiv, and dozens more).

How does the detector work?

It reads link wrappers, SMTP headers, tracking pixel hosts, and the unsubscribe URL. Combined, these identify 30+ ESPs with high confidence.

Can it spot ESP migrations?

Yes. Signed-in users get a migration timeline showing exactly when a tracked brand switched ESPs.

What if a brand uses a custom platform?

If no fingerprint matches, the detector reports "custom in-house" with the SMTP server domain so you still know where the mail came from.

Do you read the body of the email?

No private content is inspected for the ESP detection. Only public routing and link signals.

Is it free?

Yes - the lookup is free and unlimited within reasonable rate limits.

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