Formulas and metrics

Bounce Rate

The percentage of sent emails that could not be delivered to the recipient's inbox.

Tomba Team
March 23, 2026

Bounce rate in email outreach refers to the proportion of emails that are returned to the sender because they could not be delivered. It is calculated by dividing the number of bounced emails by the total number of emails sent, then multiplying by 100. A high bounce rate is a critical warning sign that can damage your sender reputation and derail your entire outreach strategy.

There are two types of bounces: hard bounces and soft bounces. Hard bounces occur when the email address is invalid, does not exist, or the domain is inactive. These are permanent failures and the address should be removed from your list immediately. Soft bounces are temporary issues, such as a full inbox or a server that is temporarily unavailable. While soft bounces may resolve on their own, repeated soft bounces to the same address should also trigger removal.

Keeping bounce rates low is essential for maintaining email deliverability and sender reputation. Industry best practice is to keep your bounce rate below 2%. By verifying your prospect lists upfront, you protect your domain reputation and ensure your outreach reaches real people.

Key Points

  • Calculated as (bounced emails / total sent) x 100
  • Hard bounces are permanent failures; soft bounces are temporary
  • High bounce rates damage sender reputation and deliverability

How It Works

When an email cannot be delivered, the receiving mail server sends a bounce notification back to the sender. Email service providers track these bounces and factor them into your sender reputation score. Too many bounces signal that you are sending to low-quality lists, which can lead to your emails being blocked entirely.

Best Practices

  • Remove hard bounces from your lists immediately after they occur
  • Aim to maintain a bounce rate below 2% across all campaigns

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