What metrics show a successful warm-up?

Multiple metrics indicate warmup progress. Monitor them consistently.

Primary indicators:

Gmail Postmaster Tools reputation: Should show medium to high, not low or bad

Inbox placement: Test emails should land in inbox, not spam

Bounce rate: Should stay below 2% (ideally under 1%)

Complaint rate: Should stay below 0.1%

Engagement signals:

Open rates: Warmup emails should show high opens (recipients are engaged contacts)

Reply rates: Some replies indicate real engagement

No spam reports: Zero complaints during warmup is the target

Delivery signals:

No rate limiting errors (421 responses)

No blocklist appearances

Consistent delivery without deferrals

What success looks like:

Stable or improving reputation scores over weeks

Ability to increase volume without metric degradation

No sudden drops in deliverability

Positive trajectory across all measured indicators

If any metric declines, pause volume increases and investigate before continuing.