Email Bounce Types

A bounced email is one that fails to reach the recipient's server and is returned. Understanding the difference between bounce types helps you troubleshoot delivery issues and maintain good sender reputation.

General Rule of Thumb

Code Pattern	Type	Meaning
4.x.x	Soft Bounce	Temporary failure - "Something went wrong temporarily. Try again later."
5.x.x	Hard Bounce	Permanent failure - "This will not work. Do not try again."

Soft Bounce Reasons

Soft bounces are temporary issues. ImprovMX will automatically retry delivery for soft bounces.

Mailbox Full (Code: 4.2.2)

The recipient's mailbox has reached its storage capacity. ImprovMX will automatically wait and resend the email later once the recipient clears space.

Server Down or Overloaded (Codes: 4.4.2, 4.3.1)

The recipient's email server is temporarily unavailable due to outages or high load. Automatic retry occurs when the server comes back online.

Email Message Too Large (Code: 5.2.3)

The email exceeded the size limits set by the recipient's server. To resolve this:

• Reduce the size of attachments
• Compress images before sending
• Use file-sharing services for large files instead of attachments

Note: While 5.2.3 starts with 5, it's often treated as a soft bounce since resending with a smaller message can succeed.

Hard Bounce Reasons

Hard bounces are permanent failures. These addresses should be removed from your contact list to protect your sender reputation.

Invalid Email Address (Code: 5.1.1)

The email address doesn't exist or cannot receive mail. Common causes include:

• Typos in the email address
• The mailbox was deleted
• The user no longer exists at that organization

Domain Doesn't Exist (Code: 5.1.2)

The domain portion of the email address (after the @) doesn't exist or has expired. This is a permanent delivery failure requiring address removal.

Recipient Server Blocked Delivery (Codes: 5.7.1, 5.5.4)

The recipient's server policy prevents acceptance of the email. This may be due to:

• Content filtering policies
• Sender IP reputation issues
• Missing or invalid authentication (SPF/DKIM)

Resolution may require contacting the recipient directly or removing the address from your list.

Best Practices

• Monitor bounce rates - High bounce rates can damage your sender reputation
• Remove hard bounces immediately - Continuing to send to invalid addresses hurts deliverability
• Let soft bounces retry - ImprovMX handles automatic retries for temporary issues
• Check your email logs - Review bounce details in your ImprovMX dashboard