Understanding bounce rates and email deliverability issues

Last updated: May 4, 2026

Bounce rates and open rates are the two most reliable early indicators of deliverability problems. Here's how to interpret them and what to do when they're off.

High bounce rate (above 2%)

What it means: A large percentage of your contact list has invalid or undeliverable email addresses.

Common causes:

  • Unverified list from a third-party source

  • Outdated contact data (people change jobs, companies shut down)

  • Manually entered addresses with typos

What to do:

  • Pause the campaign immediately if bounce rate exceeds 3%

  • Run the contact list through an email verification tool (e.g. NeverBounce, ZeroBounce) to identify and remove invalid addresses

  • For future campaigns, use Stamina's built-in enrichment — it only charges credits for verified emails, which filters out invalid addresses before sending

Low open rate (below 20%)

What it means: Emails are likely landing in spam, or the subject lines are not compelling enough to get opens.

Common causes:

  • Sending from inboxes that haven't completed warmup

  • Authentication failures (SPF, DKIM, or DMARC not passing)

  • Subject lines triggering spam filters

  • Audience mismatch — targeting the wrong persona

What to do:

  • Verify warmup status — inboxes under 3 weeks old should not run at full campaign volume

  • Test an email at Mail Tester to check your spam score and identify specific issues

  • Review subject lines for trigger words: "free", "guaranteed", "no obligation", "click here", excessive caps or punctuation

  • Reduce HTML formatting — plain or lightly formatted emails typically perform better for cold outreach

100% open rate

What it means: Almost certainly a tracking artifact, not real opens.

Cause: Email security gateways at the recipient's company pre-scan all incoming links and images, triggering your open tracking pixel for every email delivered. This is common with corporate email environments (especially Microsoft 365 with Defender).

What to do: Disable open tracking in campaign settings and rely on reply rate as your primary engagement metric instead. Reply rate is not affected by security gateway scanning.