When sending emails, encountering bouncebacks can be frustrating. Understanding email bounce status codes is crucial to diagnosing delivery issues and improving email deliverability. In this comprehensive guide, we will demystify email bounce status codes, explain their meanings, and provide practical strategies to overcome bounce-related challenges.

What Are Email Bounce Status Codes?

Email bounce status codes are numerical or alphanumeric codes that provide information about the status of an email delivery attempt. When an email bounces, the receiving mail server generates a bounce notification and includes a bounce status code. These codes help identify the reason behind the bounce and guide senders on how to address the issue.

Common Email Bounce Status Codes

1. 550 - Requested Action Not Taken: This code indicates a permanent failure, also known as a hard bounce. It typically occurs when the recipient's email address is invalid or does not exist. To resolve this, verify the recipient's email address and update your email list accordingly.

2. 421 - Service Not Available: This code represents a temporary failure, often due to a server overload or a temporary issue with the recipient's mail server. Retry sending the email after a period of time.

3. 554 - Transaction Failed: This code signifies a general failure, often due to spam filtering or content-related issues. It can indicate that the recipient's mail server blocked the email due to suspected spam or other policy violations. Review your email content and ensure compliance with email best practices.

4. 450 - Requested Mail Action Not Taken: This code indicates a temporary failure, such as the recipient's mailbox being full. The email delivery can be retried at a later time.

5. 550 - User Unknown: This code suggests that the recipient's email address is not recognized by the mail server. Verify the recipient's email address and update your contact list if necessary.

Soft Bounces vs. Hard Bounces

Soft bounces are temporary delivery failures that can be caused by issues like a full mailbox or a temporary server problem. The email delivery can be retried for soft bounces.

Hard bounces are permanent delivery failures that occur when an email cannot be delivered due to reasons like an invalid or non-existent email address. Hard bounces should be removed from your email list as continuing to send to these addresses can harm your sender reputation.

Strategies to Reduce Bounce Rates

1. Maintain a Clean Email List: Regularly clean your email list by removing invalid or inactive email addresses. Use email validation tools to verify the authenticity of email addresses before adding them to your list.

2. Use Double Opt-In: Implement a double opt-in process where subscribers confirm their email addresses before being added to your list. This helps ensure that only valid and engaged subscribers are included.

3. Monitor Email Delivery Metrics: Keep track of your email delivery metrics, including bounce rates, open rates, and click-through rates. Identify patterns and address issues promptly to maintain a healthy email deliverability rate.

4. Follow Email Best Practices: Adhere to email best practices, such as using a reputable email service provider, personalizing your emails, and avoiding spam trigger words or deceptive subject lines.


Understanding email bounce status codes is essential for email marketers and senders. By familiarizing yourself with common bounce codes and implementing strategies to reduce bounce rates, you can improve email deliverability, enhance campaign performance, and maintain a positive sender reputation. Regularly monitor your email delivery metrics, keep your email list clean, and follow best practices to ensure your emails reach the intended recipients' inboxes.