How Does ESP-Aware Routing Work?
Superkabe scores mailboxes by their actual performance per recipient ESP and pins the best ones for each lead
Quick Answer
When a lead is routed to a campaign, Superkabe classifies the recipient's email provider (Gmail, Microsoft, Yahoo) via MX lookup, then scores each mailbox in the campaign based on its 30-day bounce rate to that specific ESP. The top 3 performing mailboxes are pinned for this lead via assigned_email_accounts. The sending platform only uses those mailboxes for this specific lead.
This goes beyond simple ESP matching. A Gmail mailbox with a 2% bounce rate to Gmail recipients gets a low score — an Outlook mailbox with 0.1% bounce rate to Gmail gets a high score. Performance beats provider matching.
How ESP Classification Works
During email validation, Superkabe resolves the recipient domain's MX records and classifies the email provider:
| MX Pattern | ESP Bucket | Example domains |
|---|---|---|
| *.google.com, *.gmail.com | Gmail | gmail.com, any Google Workspace domain |
| *.outlook.com, *.microsoft.com | Microsoft | outlook.com, hotmail.com, any M365 domain |
| *.yahoodns.net | Yahoo | yahoo.com, aol.com |
| Everything else | Other | Self-hosted, Proofpoint, Mimecast |
Classification is cached per domain in the DomainInsight table. One lookup per domain, reused for all leads at that domain.
How Mailbox Scoring Works
1. Data Collection
Every email sent, bounced, and replied generates a tracking event tagged with the sending mailbox and the recipient's ESP. These events accumulate over 30-day rolling windows.
2. Performance Aggregation
Every 6 hours, the ESP performance worker aggregates events into a per-mailbox per-ESP matrix: send count, bounce count, reply count, and computed bounce rate for the last 30 days.
3. Scoring at Route Time
When a lead is pushed to a campaign, Superkabe scores each campaign mailbox against the recipient's ESP. Lower bounce rate = higher score. A volume confidence bonus rewards mailboxes with more data. Top 3 mailboxes are selected.
4. Mailbox Pinning
The selected mailbox IDs are passed to Smartlead via assigned_email_accounts. Smartlead will only use those 3 mailboxes for this specific lead. Its own ESP matching and load balancing run within this restricted pool.
What Does “Warming Up” Mean?
ESP scoring requires at least 30 sends per mailbox per ESP bucket before the score is considered reliable. Until that threshold is reached, the cell shows “warming up” in the ESP Performance Matrix and scoring is skipped — the sending platform picks the mailbox instead.
At 10,000 sends/month with 20 mailboxes and 4 ESP buckets, you'll have reliable data within 3–4 weeks of normal sending.
Reading the ESP Performance Matrix
The matrix on the Email Validation page shows each mailbox's bounce rate per ESP:
- <1% bounce rate — healthy. This mailbox performs well for this ESP.
- 1–2% bounce rate — warning. Monitor closely, may degrade further.
- >2% bounce rate — problematic. Superkabe avoids routing this ESP's leads to this mailbox.
- warming up — fewer than 30 sends in this cell. Not enough data to score.