What Is an Invalid Hard Bounce?

An Invalid Hard Bounce occurs when you attempt to send email to an address that does not exist or is otherwise undeliverable at the receiving mailbox provider (MBP). The MBP returns a permanent failure (5xx) response indicating the address is invalid. Future attempts to send to that address will always fail.

High rates of invalid hard bounces are a strong indicator of poor list quality, poor data collection, or risky marketing practices. Excessive invalid bounce rates negatively impact your domain and IP reputation and can reduce inbox placement.

While spam complaint rates have established industry standards, no universal benchmark exists for invalid hard bounces. At Keap, an invalid hard bounce rate above 1-2% per mailbox provider is considered excessive and may prompt outreach from the Email Compliance Team.

How Invalid Addresses End Up on Your List

1. Sending to a Cold or Aged Email List

Email data decays rapidly—~30% of people change their email each year.
If you have not sent to contacts in 4+ months, many may have changed addresses without resubscribing, leading to invalid bounces when you resume sending.

Regular, consistent sending helps maintain list hygiene and reduces undetected decay.

2. Purchased or Scraped Lists

Purchased or scraped data often contains high percentages of invalid, unverified, or stale email addresses.

• These lists typically cause severe bounce and complaint issues.

• Keap prohibits the use of purchased or scraped lists.

• Accounts discovered using them are at risk of suspension.

3. Poor Lead Generation Practices

Fake or mistyped addresses are common in forms where users:

• Want gated content but do not trust the brand yet.

• Are testing your form.

• Mistype their email intentionally or accidentally.

4. List Bombing

List bombing occurs when bots fill out your web forms using massive numbers of random or malicious email addresses.
This can result in:

• Large volumes of invalid bounces

• Spam traps

• Abuse complaints

Sites without proper form protection (reCAPTCHA, rate-limiting, bot detection) are highly vulnerable.

Why Invalid Hard Bounces Matter

Invalid bounces signal systemic issues in your email acquisition or maintenance process. High invalid rates:

• Damage your sender reputation

• Increase spam filtering

• Reduce inbox placement across major MBPs

• Risk compliance intervention

• Indicate poor list hygiene and unreliable data collection

Maintaining low invalid bounce rates is essential to long-term deliverability.

Resolution: 

Required Actions to Reduce Invalid Hard Bounces

If you have been contacted by Keap’s Email Compliance Team regarding excessive invalid hard bounces, please complete the following remediation steps.

1. Perform List Hygiene

A. Remove Contacts Unengaged for Over 12 Months

1. Log in to your Keap application.

2. Navigate to Reports:

   • For Pro / Max users: go to Reports → Email Engagement Tracker.

   • For Classic users: go to Marketing → Reports → Email Status Search.

3. Sort by column Time Since Last Engagement.

4. Select contacts unengaged >12 months.

5. Go to Actions > Update Opt-In/Out Status → Opt-Out → Process Action.

These contacts are no longer viable and pose high bounce and complaint risk.

B. Flag Contacts Unengaged for 6–12 Months

1. Navigate back to Reports:

   • For Pro / Max users: go to Reports → Email Engagement Tracker.

   • For Classic users: go to Marketing → Reports → Email Status Search.

2. Sort by column Time Since Last Engagement.

3. Select contacts unengaged for 6–12 months.

4. Go to Actions > Apply Tag.

5. Create a tag such as “Bad Engagement” and apply it.

6. Build a list for review:

   • Go to Contacts > Contact Lists > Create a List

   • Add filter: Other Fields > Tags > Includes Any

   • Select the “Bad Engagement” tag

   • Name the list (e.g., Clean Up Engagement)

7. For each contact:

   • Review the contact record.

   • If they should no longer receive marketing emails:

     • Remove broadcast and automation tags (Other > Tags)

     • Remove them from any active automations

     • Opt out the contact’s email address

These contacts may still be salvageable with targeted re-engagement but should not be included in broadcasts until they re-engage.

2. Audit Your Lead Collection Workflow

A. Audit Landing Pages and Web Forms

Check all forms for:

• Missing CAPTCHA

• No bot protection

• URL exposure that allows automated submissions

• Incorrect validation rules

• Lack of double opt-in (recommended for high-risk forms)

Fixing form vulnerabilities prevents list bombing and invalid sign-ups.

B. Evaluate Lead Sources

Determine where invalid addresses are entering:

• Third-party integrations

• Events

• Imports

• API submissions

• Old or unmonitored landing pages

Ensure all collection points follow best practices and enforce basic validation.

C. Strengthen Trust and Transparency

Users provide valid email addresses when they trust the sender. Improve:

• Branding and page security signals

• Clear value statements on forms

• Visible privacy assurances

• Reduced friction in subscription processes

3. Review and Implement Email Best Practices

Key areas include:

• Regular sending cadence

• Engagement-based segmentation

• Double opt-in for high acquisition risk scenarios

• Avoiding large reactivation blasts to cold lists

• Continuous data hygiene maintenance

Summary

Invalid hard bounces are a critical deliverability issue caused by poor data quality, risky acquisition practices, or inadequate list hygiene. Keeping invalid bounce rates low protects your sender reputation and improves inbox placement.

If contacted by Keap Compliance, prompt corrective action is required to prevent further degradation of your email reputation.