How do multi-ESP setups duplicate metrics?
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If you're routing email through more than one platform, your engagement data is probably being counted more than once. Not because anyone is doing anything wrong, just because that's how tracking works when there are multiple observers.
The most common scenario: a marketing team uses Mailchimp for newsletters and Postmark for transactional email, with everything also passing through a CRM like HubSpot. If all three platforms embed their own tracking pixels or track link clicks through their own redirect domains, the same subscriber interaction can register in three different places as three separate events.
The duplication gets worse when you add a dedicated deliverability layer. If you're using a tool like Validity or routing through a custom SMTP relay that adds its own tracking, every pixel load now hits multiple systems. A single open might show up in your ESP, your CRM, and your deliverability monitoring platform simultaneously.
How to reduce the problem: standardize on one source of truth per metric type. Let your ESP own email event data (opens, clicks, bounces). Let your CRM own post-click behavior (page views, purchases, lead status). Use unique message identifiers to tie records together when you need to reconcile across systems rather than comparing raw counts.
Time zone alignment matters too. If one system logs in UTC and another in local time, the same event at 11:45 PM UTC might land in different days across your reports. Standardize everything to UTC at the point of capture.
For a framework to reconcile conflicting numbers between systems, see how to reconcile conflicting data sources. And if you're not sure which metrics to trust for deliverability vs. campaign performance, that covers the split clearly.
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