ESP-Aware Mailbox Routing
Last updated: April 24, 2026
Superkabe scores every mailbox by 30-day per-ESP performance — bounce rate to Gmail, bounce rate to Microsoft, bounce rate to Yahoo, bounce rate to Other — and pins the top 3 mailboxes for each lead based on recipient ESP. An Outlook mailbox with 0.1% Gmail bounce rate outscores a Gmail mailbox with 2% Gmail bounce rate.
Superkabe routes leads to the best-performing mailboxes for each recipient's email provider. Most sending platforms offer simple ESP matching: Gmail mailbox to Gmail recipient. Superkabe goes further. It tracks every send, bounce, and reply per mailbox per ESP over a 30-day rolling window. A Gmail mailbox with a 2% bounce rate to Gmail gets skipped. An Outlook mailbox with 0.1% bounce rate to Gmail gets selected. Performance beats provider matching.
How ESP-Aware Routing Differs from ESP Matching
Smartlead and Instantly offer basic ESP matching: route Gmail recipients to Gmail mailboxes. This is a binary rule. If you have 5 Gmail mailboxes, the platform picks one at random. It does not know which of those 5 performs best with Gmail recipients.
Superkabe tracks actual performance data. Every email sent creates a SendEvent tagged with the sending mailbox and the recipient ESP. Every bounce and reply creates corresponding events. Every 6 hours, these events are aggregated into a per-mailbox per-ESP performance matrix.
When a new lead is routed, Superkabe queries this matrix. A mailbox with 500 sends to Gmail and a 0.2% bounce rate scores higher than a mailbox with 50 sends and 0% (less data, lower confidence). The top 3 mailboxes are pinned to the lead via assigned_email_accounts.
The Performance Matrix
The ESP Performance Matrix on the Email Validation dashboard shows every mailbox in your account with bounce rates broken down by Gmail, Microsoft, Yahoo, and Other. Color-coded: green under 1%, yellow 1-2%, red above 2%.
Cells with fewer than 30 sends show 'warming up' instead of a bounce rate. This prevents routing decisions based on noise. After 3-4 weeks of normal sending volume, most cells will have enough data for reliable scoring.
Use this matrix to identify mailboxes that underperform for specific ESPs. A mailbox might be excellent for Outlook (0.3% bounce rate) but poor for Gmail (2.1%). Without per-ESP breakdown, this pattern is invisible in aggregate stats.
Works Across All Ingestion Paths
ESP-aware scoring runs on every lead entry path. CSV uploads through the Lead Control Plane, Clay webhook ingestion, and direct API calls all classify the recipient ESP and score mailboxes before pushing.
For EmailBison, which lacks native ESP matching, Superkabe is the only layer providing any form of ESP-aware routing. The assigned_email_accounts parameter is Smartlead-specific today, but the scoring data is available for all connected platforms.
Frequently Asked Questions
Why performance-based and not naive ESP-to-ESP matching?
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Because ESP alignment is a proxy, not a cause. What actually matters is the historical performance of the mailbox against the recipient's ESP — and that varies by mailbox, not by provider class. Performance-based scoring captures the signal directly.
How many sends before ESP scoring activates?
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30 sends per (mailbox, ESP) cell. Below that, the sample is too small to score reliably — those cells are handled by the warming-up fallback distribution.