How to Find Out What ESP Any Company Uses (Free Method + Tool)

Knowing which email service provider a company uses tells you a lot: how sophisticated their sending infrastructure is, whether they are scaling or cutting costs, and which tool gaps you can compete on. The answer is hiding in every email they send - you just need to know where to look.

How to Find What ESP a Company Uses: 5 Steps

1. Step 1

   Open a newsletter from the target company

   Subscribe to - or locate in your inbox - a recent marketing or newsletter email from the company you want to research. A promotional or newsletter email works best; transactional emails (receipts, password resets) sometimes use a different ESP than the marketing stack. Make sure you have the full message open in your email client before proceeding.

2. Step 2

   View the email source or raw headers

   Every major email client exposes the raw message source:

   • Gmail: open the email, click the three-dot menu (top-right), choose Show original.
   • Outlook: open the email, go to File, then Properties - the headers appear in the Internet headers box.
   • Apple Mail: View menu, then Message, then All Headers.
   • Fastmail / HEY: look for "View source" or "Raw message" in the message actions menu.
3. Step 3

   Look for DKIM selector patterns

   In the raw source, search (Ctrl+F / Cmd+F) for DKIM-Signature. The s= field is the selector and the d= field is the signing domain. ESPs use predictable patterns:

   • s=s1 with d=sendgrid.net - SendGrid
   • s=k1 with d=mailchimp.com - Mailchimp
   • s=klaviyo with d=klaviyo.com - Klaviyo

   If the brand uses custom domain authentication, the d= tag will show their own domain - move on to Step 4 to confirm via headers.

4. Step 4

   Check the Return-Path and X-Mailer headers

   Search the raw source for Return-Path and X-Mailer (or X-Originating-IP). These reveal the bounce-handling domain and occasionally the platform name outright:

   • Return-Path: <bounce@em.sendgrid.net> confirms SendGrid
   • Return-Path: <bounce-...@bounce.list-manage.com> confirms Mailchimp
   • X-Mailer: HubSpot confirms HubSpot
   • Return-Path: ...@em.beehiiv.com confirms Beehiiv
5. Step 5

   Use the Newsletrix ESP Detector to automate steps 2-4

   Doing this manually works for one or two brands. For a list of competitors - or if the headers are ambiguous - use the Newsletrix ESP Detector. Paste or upload the raw message source and the tool cross-references DKIM selectors, Return-Path domains, link-wrapper hosts, tracking pixel URLs, and UTM conventions across 30+ known ESPs to return a confident match in seconds.

Common ESP Fingerprints: DKIM Selector Reference

Use this table as a quick-reference when reading headers manually. The DKIM selector is the most reliable single signal for platforms that do not use custom domain authentication.

ESP	Typical DKIM Selector Pattern	Signing Domain
SendGrid	s=s1 or s=s2	sendgrid.net
Mailchimp	s=k1	mailchimp.com
HubSpot	s=hs1- prefix	hubspotemail.net
Klaviyo	s=klaviyo	klaviyo.com
ConvertKit / Kit	s=ck1	convertkit-mail.com
ActiveCampaign	s=ac- prefix	activehosted.com
Beehiiv	s=beehiiv	beehiiv.com

Note: when a sender configures custom domain authentication, the d= field shows their own domain rather than the ESP's. In those cases, fall back to Return-Path domain, link-wrapper host, and tracking pixel URL patterns as described in Steps 3 and 4. For a deeper walkthrough of all seven fingerprinting signals, see How to spot which ESP a competitor uses (from the email alone).

Why Knowing a Competitor's ESP Matters

The ESP choice reveals more than tech stack preferences. It signals budget tier, team sophistication, and growth trajectory. A solo creator moving from Mailchimp to Beehiiv is optimising for monetisation. A mid-market brand migrating from Klaviyo to Iterable is usually scaling toward enterprise lifecycle programs. Catching that shift when it happens - before the brand announces it - gives you a head start on outreach, positioning, and competitive strategy.

For a structured approach to analysing competitor newsletters beyond just the ESP, see How to analyse competitor email marketing.

Frequently Asked Questions

How do I check what email software a company uses?

Open a newsletter from the company and view the raw message source. Search for DKIM-Signature and match the selector and signing domain against known ESP fingerprints. The Return-Path header and X-Mailer field provide backup confirmation. For an automated result, paste the raw headers into the Newsletrix ESP Detector.

Can you tell what ESP from email headers?

Yes. The DKIM-Signature header contains a selector (s=) and signing domain (d=) that identify the platform in most cases. The Return-Path domain and X-Mailer header add further confirmation. Together these fields are enough to identify the ESP with high confidence for brands that have not configured fully custom domain authentication.

What is the DKIM selector method for ESP detection?

The DKIM selector is the s= value in the DKIM-Signature header. ESPs assign predictable selector names - SendGrid uses s1, Mailchimp uses k1, Klaviyo uses klaviyo. Matching that selector against a lookup table of known patterns is the fastest manual method for ESP identification and is more reliable than link-wrapper inspection for brands with custom click-tracking domains.

Is there a free tool to detect the ESP of a newsletter?

Yes. The Newsletrix ESP Detector is free to use. Paste or upload the raw message source from any email client and the tool returns the ESP name, the specific signals used for detection (DKIM selector, Return-Path, link wrapper), and a confidence level. It covers 30+ ESPs including all major platforms and several niche ones.

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