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Understanding Email Bounces

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An email bounce (also called a bounce message or non-delivery report) is a notification that an email could not be delivered to its intended recipient. Bounces are returned to the sender (or the Return-Path address) by the receiving mail server.

What is an email bounce?

An email bounce occurs when an email cannot be delivered to its recipient. The receiving mail server sends a bounce message back to the sender explaining why delivery failed.

Types of email bounces

Hard bounces

Hard bounces are permanent delivery failures. These indicate that the email will never be delivered to the recipient.

Common causes:

  • Invalid email address: The email address does not exist
  • Domain does not resolve or does not exist: The recipient’s domain is not registered or has no valid DNS records.
  • Blocked domain: The recipient’s domain blocks all emails
  • Recipient mailbox does not exist: The mailbox was deleted or never existed

Characteristics:

  • Permanent failure
  • Should be removed from email list immediately
  • High hard bounce rates can damage sender reputation
  • Cannot be retried (will always fail)

Example bounce messages:

  • “550 5.1.1 User unknown”
  • “550 5.1.2 Host unknown”
  • “550 5.4.1 Recipient address rejected”

Soft bounces

Soft bounces are temporary delivery failures. These indicate that the email might be deliverable in the future.

Common causes:

  • Mailbox full: The recipient’s mailbox is full
  • Server temporarily unavailable: The recipient’s mail server is down or busy
  • Message too large: The email exceeds size limits
  • Temporary block: The recipient’s server temporarily blocked the email
  • Greylisting: The server is using greylisting (delayed delivery)

Characteristics:

  • Temporary failure
  • May be retried by the sending server
  • Should be monitored but not immediately removed from list
  • Can become hard bounces if persistent

Example bounce messages:

  • “452 4.2.2 Mailbox full”
  • “451 4.3.0 Temporary server error”
  • “552 5.2.2 Mailbox full”

Bounce categories

Delivery status notification (DSN) codes

Bounces include DSN codes that indicate the type of failure:

5.1.x - Addressing errors:

  • 5.1.1: Bad destination mailbox address
  • 5.1.2: Bad destination system address
  • 5.1.3: Bad destination mailbox address syntax

5.2.x - Mailbox errors:

  • 5.2.1: Mailbox disabled
  • 5.2.2: Mailbox full
  • 5.2.3: Message length exceeds administrative limit

5.3.x - Mail system errors:

  • 5.3.1: Mail system full
  • 5.3.2: System not accepting network messages
  • 5.3.3: Mail system not capable of features

5.4.x - Network errors:

  • 5.4.1: Host not found
  • 5.4.2: Bad connection
  • 5.4.3: Routing server failure

5.5.x - Protocol errors:

  • 5.5.1: Invalid command
  • 5.5.2: Syntax error
  • 5.5.3: Too many recipients

5.7.x - Security/Policy errors:

  • 5.7.1: Delivery not authorized
  • 5.7.2: Mailing list expansion prohibited
  • 5.7.3: Security conversion required

Note

The first digit (5) indicates permanent failure, while (4) indicates temporary failure. The second digit indicates the category of failure.

Understanding bounce messages

Bounce message structure

Bounce messages typically include:

  1. Original message: The original email that could not be delivered
  2. Bounce reason: Explanation of why delivery failed
  3. DSN or SMTP status: Status code indicating the type of failure
  4. Diagnostic information: Additional details about the failure

Reading bounce messages

Subject line: Often contains “Delivery Status Notification” or “Undelivered Mail”

Body: Contains:

  • The reason for the bounce
  • DSN codes
  • The original email (or headers)
  • Diagnostic information

Headers: May contain additional information about the bounce

Common bounce scenarios

Invalid email address

Scenario: Email address does not exist

Bounce message: “550 5.1.1 User unknown”

Action: Remove from email list (hard bounce)

Mailbox full

Scenario: Recipient’s mailbox is full

Bounce message: “552 5.2.2 Mailbox full”

Action: Retry later (soft bounce), remove if persistent

Domain does not exist

Scenario: Recipient’s domain does not exist

Bounce message: “550 5.1.2 Host unknown”

Action: Remove from email list (hard bounce)

Server temporarily unavailable

Scenario: Recipient’s mail server is down

Bounce message: “451 4.3.0 Temporary server error”

Action: Retry later (soft bounce)

Message rejected by recipient

Scenario: Recipient’s server rejected the email

Bounce message: “550 5.7.1 Delivery not authorized”

Action: Investigate why, may need to remove from list

Impact of bounces on deliverability

High bounce rates

High bounce rates can:

  • Damage sender reputation: High bounce rates indicate poor list quality
  • Trigger spam filters: ISPs may flag high bounce rates as spam
  • Get you blacklisted: Very high bounce rates can result in blacklisting
  • Reduce deliverability: Poor reputation reduces inbox placement

Best practices for bounce rates

  • Hard bounces: Keep below 0.5%
  • Soft bounces: Keep below 2%
  • Total bounce rate: Keep below 2%

Handling bounces

For hard bounces

  1. Remove immediately: Remove hard bounces from your email list immediately
  2. Do not retry: Do not attempt to resend to hard bounce addresses
  3. Monitor rate: Keep track of hard bounce rates
  4. Investigate: If hard bounce rate is high, investigate list quality

For soft bounces

  1. Retry: Allow your email service to retry soft bounces
  2. Monitor: Monitor soft bounces to see if they become hard bounces
  3. Remove if persistent: Remove addresses that consistently soft bounce
  4. Investigate: If many soft bounces, investigate recipient servers

Best practices

  • Remove hard bounces immediately
  • Monitor bounce rates regularly
  • Use double opt-in to reduce bounces
  • Validate email addresses before sending
  • Clean your email list regularly
  • Monitor bounce trends

Bounces and email forwarding

When using email forwarding:

Bounces from forwarded emails

Scenario: An email is forwarded, but the final destination bounces

What happens:

  • The bounce may come back to the forwarding service
  • The bounce may come back to the original sender
  • Behavior depends on the forwarding configuration

Considerations:

  • Bounces cannot be forwarded to the original sender in most cases
  • The forwarding service may handle bounces
  • Check your email forwarding service’s bounce handling

Note

For more information about handling bounces with email forwarding, see Handling Email Bounces with Email Forwarding.

Reducing bounces

List hygiene

  • Validate addresses: Use email validation services
  • Double opt-in: Require confirmation for new subscribers
  • Regular cleaning: Remove inactive addresses periodically
  • Monitor bounces: Track and remove bounces promptly

Email practices

  • Respect size limits: Keep emails under size limits
  • Follow best practices: Follow email sending best practices
  • Monitor reputation: Keep sender reputation high
  • Test before sending: Test emails before sending to large lists

Monitoring bounces

Track bounce rates

  • Hard bounce rate: Percentage of hard bounces
  • Soft bounce rate: Percentage of soft bounces
  • Total bounce rate: Combined bounce rate
  • Bounce trends: Track changes over time

Set up alerts

  • High bounce rate alerts: Get notified when bounce rates exceed thresholds
  • Hard bounce alerts: Get notified of hard bounces immediately
  • Trend alerts: Get notified of unusual bounce patterns

Have more questions?

If you have additional questions or need any assistance with understanding email bounces, just contact support, and we’ll be happy to help.