Competitor Inbox fingerprints 102 email service providers using a multi-signal detection engine — DKIM signing domains, Return-Path patterns, proprietary headers, and tracking infrastructure. No manual tagging. No guesswork. Every email classified automatically.
Knowing which ESP a competitor uses — and whether they've switched — is one of the most actionable signals in competitive email intelligence. Switching ESPs is a major operational event. It signals budget shifts, deliverability problems, scaling decisions, and sometimes a complete martech overhaul.
Get alerted the moment a competitor changes their sending platform — often a leading indicator of strategic pivots.
Compare ESP choices across your competitive set. See if market leaders all gravitate toward the same platforms.
See what top-performing brands in your industry use. Let real market data guide your ESP evaluation.
Every email captured in your competitor inbox is tagged with the detected ESP at parse time — no manual work required.
Our multi-signal fingerprinting engine evaluates four independent evidence sources, then weights and combines them into a confidence score for every email.
Every ESP signs outbound mail with their own DKIM domain (e.g. d=klaviyomail.com). This is the most reliable signal — like a cryptographic fingerprint baked into every email.
The Return-Path header reveals the envelope sender domain used by the ESP for bounce handling. Each provider uses a distinct subdomain pattern (e.g. bounce.em.klaviyo.com).
Most ESPs inject vendor-specific headers like X-SG-EID (SendGrid), X-Mailer-RecptId (HubSpot), or X-Mailgun-Tag into every message. We check for these patterns on every email.
Click-tracking URLs, open-tracking pixels, and List-Unsubscribe links all point to ESP-owned domains. These are cross-referenced against our database of known ESP CDN and tracking infrastructure.
Each signal carries a weight. DKIM domain matches score highest (deterministic — cryptographically tied to the ESP), followed by Return-Path domains, proprietary headers, and tracking link patterns. If any deterministic signal fires, the result is immediately labelled "High" regardless of total score. The engine normalises across all fired signals into a 0–100 confidence value so you can quickly distinguish a firm identification from a likely match.
From the biggest marketing automation platforms to boutique newsletter tools — we fingerprint them all. New ESPs are added continuously as we discover new sending infrastructure patterns.
Don't see an ESP you need?
We're constantly expanding our fingerprint database. If you're monitoring a sender and the ESP isn't being detected, let us know and we'll add it.
Request an ESPStart a free trial and get automatic ESP detection, DNS auth scoring, creative tracking, and send-pattern intelligence — all in one place.