MCP

Core metrics

TL;DR

  • Reply rate is the primary metric. It is the only one that confirms a human read the email and cared enough to act.
  • Delivery and bounce rates are health checks; open rate is broken; positive-reply rate is what you actually want but is the noisiest.
  • Always record the denominator. The same campaign can honestly be described as 0.45% or 3.4% depending on what you divide by.

The primary metric

Reply rate. A reply requires human intent, which is what makes it the one signal that survives scrutiny — opens are inflated by mail clients, security scanners and preview panes that fire tracking pixels without a person involved (2026) [1].

Two things must be fixed before a reply rate means anything: what counts as a reply (all replies, including objections and unsubscribes — or only positive ones), and what you divide by (unique prospects contacted, or total emails sent across the sequence). Both are legitimate; mixing them between campaigns is how teams accidentally conclude that a change worked.

The health checks

Delivery and bounce rate. Not performance metrics — safety metrics. A 20M+ email corpus puts the average bounce rate at 5.1%, with under 2% rated good (2026) [2]; operating guidance says pause and clean above 2% [3]. Read this daily during a send, not weekly (Campaign scheduling and pausing).

Spam-complaint rate. The number with the least headroom in the whole system, and the one you usually cannot see directly without provider tooling. Treat any visible complaint as significant.

Unsubscribe rate. Partly a compliance signal, partly a targeting one. A rising unsubscribe rate with a flat reply rate means you are reaching the wrong people, not writing badly.

The metric everyone quotes and nobody should

Open rate. Measured by a tracking pixel, and structurally broken since Apple's Mail Privacy Protection began pre-loading images in 2021 — with Apple Mail at roughly half the global client market, a large share of your "opens" are machines (2026) [1]. Published open rates now span 27.7-44% depending on source and how much inflation is baked in [2].

The pixel also has a cost: it is a signal filters use, and the school that recommends removing it entirely is credible (Tracking caveats).

What you actually want

Positive-reply rate and meetings booked are the metrics closest to the goal, and both are too sparse for fast feedback: at a 3% reply rate, a thousand-contact campaign produces about thirty replies and maybe ten positive ones. You can steer on reply rate weekly; you can only steer on meetings monthly, across campaigns.

Cost per meeting is the metric that decides whether the channel earns its place (Outbound math and economics).

Segment every metric

A campaign average is a blend of segments that behave differently — company size alone moves reply rates threefold [4]. Report by segment and tier, or you will optimize copy in response to a targeting effect. That is the diagnosis path in Diagnosing underperformance.

References

  1. Smartlead — Cold email open rate accuracy (vendor blog, Jun 2026)
  2. Woodpecker — Cold email statistics (20M+ email corpus, updated Jun 2026)
  3. Smartlead — Email deliverability guide (vendor guide, Jun 2026)
  4. Belkins — Cold email response rates (7.5M emails sent in 2025, updated Jun 2026)