Common Email Bounce Errors and How to Fix Them

What Are Email Bounces? #

An email bounce occurs when an email you send fails to reach the intended recipient. Instead, it is “bounced” back to you (or sometimes silently dropped) with a notification indicating a delivery failure. Bounce messages usually contain an SMTP status code and some additional details explaining why delivery was unsuccessful.

Hard vs. Soft Bounces #

• Soft Bounce
  A temporary issue like a full mailbox or a server that’s temporarily unavailable. The email server often tries to resend the message for a certain period before giving up.
• Hard Bounce
  A permanent failure, typically due to an invalid address, domain issues, or an email server blocking your message. These won’t be retried automatically.

Structure of a Bounce Message #

A typical bounce message might look something like this:

This is the mail system at host mail.yourdomain.com.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. 
The following address(es) failed:

recipient@example.com
   host example-com.mail.protection.outlook.com [123.45.67.89]
   SMTP error from remote mail server after RCPT TO:<recipient@example.com>:
   550 5.1.1 RESOLVER.ADR.RecipNotFound; not a valid recipient

Final-Recipient: rfc822; recipient@example.com
Action: failed
Status: 5.1.1
Remote-MTA: dns; example-com.mail.protection.outlook.com
Diagnostic-Code: smtp; 550 5.1.1 RECIPIENT NOT FOUND

Key Elements to Look For #

• Recipient Address: The specific email address that caused the bounce.
• SMTP Status Code (e.g., 550, 552): A numeric code that indicates the type of error.
• Diagnostic Code or Error Message: A human-readable explanation provided by the remote server, such as “Mailbox not found” or “Quota exceeded.”

Common SMTP Status Codes and Their Meanings #

Below are some of the most commonly encountered SMTP codes you may see in bounce messages, along with examples and possible solutions.

1. 550 – Requested Action Not Taken #

• Typical Error Text: “550 5.1.1 User unknown” or “Mailbox unavailable.”
• Meaning: This indicates a permanent failure. It usually means the recipient email address does not exist or is spelled incorrectly.
• Example Scenario: You send an email to john.doe@exmple.com (notice the missing ‘a’ in example). The server cannot find this mailbox, so it bounces back with a 550 error.
• How to Fix:
  • Check Spelling: Ensure the email address is typed correctly.
  • Confirm Valid Recipient: Reach out to the recipient through another channel (like phone or chat) to verify the correct email.

2. 552 – Storage Allocation Exceeded #

• Typical Error Text: “552 Message size exceeds fixed limit” or “552 Quota exceeded.”
• Meaning: The recipient’s mailbox is full, or the email message is larger than the server’s maximum allowed size.
• Example Scenario: You attempt to send a 50MB attachment, but the recipient’s mail server only accepts messages up to 25MB.
• How to Fix:
  • Ask Recipient to Free Up Space: The recipient might need to delete old messages or increase their quota.
  • Reduce Attachment Size: Compress your files or use a file-sharing service (e.g., Google Drive, Dropbox) instead of emailing large attachments.

3. 554 – Transaction Failed (or Message Rejected) #

• Typical Error Text: “554 5.7.1 Message rejected due to spam or virus content” or “554 Transaction failed.”
• Meaning: This is often due to a security or spam policy issue. The receiving server rejects the message because it detects potential spam, malware, or suspicious content.
• Example Scenario: You send a marketing email with certain phrases or links that trigger the recipient’s spam filter.
• How to Fix:
  • Check Your Content: Remove or reduce spam-trigger words, suspicious links, or large images.
  • Review Sender Reputation: Ensure your IP/domain isn’t on a blacklist. Use online tools like MXToolbox to check.
  • Implement Email Authentication: Properly configure SPF, DKIM, and DMARC so receiving servers can verify your emails are legitimate.

4. 421 – Service Not Available (Temporary) #

• Typical Error Text: “421 4.3.2 Service not available, closing transmission channel.”
• Meaning: A temporary issue with the receiving mail server. It could be overloaded or experiencing downtime.
• Example Scenario: High traffic on the recipient’s mail server causes timeouts, so your email bounces back with a 421 code.
• How to Fix:
  • Retry: This is usually a short-term problem. Wait and let your mail server retry automatically or manually resend after a few hours.
  • Contact Recipient IT: If the issue persists, the recipient’s administrator might need to check their server’s health or capacity.

5. 450 – Requested Action Not Taken (Mailbox Unavailable) #

• Typical Error Text: “450 4.2.1 Mailbox unavailable, try again later.”
• Meaning: A temporary error indicating the mailbox or recipient’s server can’t accept the message right now. This could be due to too many connections, spam filtering thresholds, or a user’s mailbox being locked for maintenance.
• Example Scenario: Your emails come in too quickly, and the server temporarily blocks additional connections.
• How to Fix:
  • Automatic Retries: Most mail servers will retry for a set period.
  • Delay Sending: If sending in bulk, slow down or stagger your sends.

6. 451 – Local Error in Processing #

• Typical Error Text: “451 Temporary server error. Please try again later.”
• Meaning: This is a local error on the receiving mail server, often due to a temporary overload, a system glitch, or spam filtering triggers.
• Example Scenario: The receiving server might be running a large antivirus scan that’s delaying message acceptance.
• How to Fix:
  • Wait and Retry: The issue might resolve itself once the server load decreases.
  • Check Sender Reputation: If your domain or IP triggers spam filtering, see if you need to improve your email practices.

7. 553 – Mailbox Name Not Allowed #

• Typical Error Text: “553 5.3.0 Address syntax is invalid.”
• Meaning: Similar to a 550 error but specifically related to improper formatting or a domain that the server refuses to handle.
• Example Scenario: You accidentally typed recipient@@example.com or an invalid domain ex@mple..com.
• How to Fix:
  • Correct the Email Address: Validate you’re using a proper address format (username@domain.tld).
  • Contact Recipient: If you suspect the domain is spelled incorrectly, verify the actual domain name.

8. 571 – Delivery Not Authorized #

• Typical Error Text: “571 Delivery not authorized, message refused.”
• Meaning: The receiving server requires some form of authentication or the sender is explicitly blocked.
• Example Scenario: A server that only accepts mail from specific IP ranges or domains might block your message if you aren’t recognized.
• How to Fix:
  • Check Any Allowlist Requirements: Ask your recipient if they need to add you to their allowlist or “safe sender” list.
  • Enable SMTP Authentication: Ensure you’re sending mail with proper credentials from your domain.

General Tips for Resolving Bounce Errors #

1. Review the Entire Bounce Message
   • Don’t just look at the code; read the descriptive text. It often provides a clue (“Mailbox not found,” “Message too large,” etc.).
2. Verify Recipient Information
   • Typos in the domain or mailbox name are common, so always double-check.
3. Check Your Email Authentication (SPF, DKIM, DMARC)
   • Having these records properly set up reduces the likelihood of being flagged as spam or rejected outright.
4. Monitor Your IP/Domain Reputation
   • Use tools (e.g., MXToolBox, SenderScore) to see if you’re blacklisted or have a poor sending reputation.
5. Use a Professional SMTP Service
   • For high-volume sending, consider dedicated email-sending services (e.g., Mailgun, SendGrid) to reduce the risk of IP blacklisting.
6. Contact Recipient’s IT or Your Email Host (AEserver)
   • If you’re stuck, share the bounce message details. Often, the receiving server’s IT can whitelist your domain or troubleshoot deeper issues on their end.
   • AEserver support can also help interpret bounce messages and check server-side settings.

Real-Life Examples #

1. Sending an Email to a Misspelled Address
   • You type sales@exmaple.com instead of sales@example.com. The server responds with 550 5.1.1 User unknown.
     Fix: Correct the spelling and resend.
2. Large Attachment Bounces
   • You send a 35MB PDF to a mailbox limited to 20MB attachments. The bounce code is 552 Quota exceeded or 552 Message size exceeds limit.
     Fix: Compress the file or use a file-sharing link.
3. Spam Filter Catch
   • You run a marketing campaign with all-caps subject lines like “BUY NOW!!!” and suspicious links. The receiving server bounces with 554 5.7.1 Message Rejected.
     Fix: Tone down the salesy language, ensure your domain passes SPF/DKIM, and possibly segment your send volume.

How to Share Bounce Information with Support #

When contacting AEserver or any support team about bounce issues, provide:

• Full Bounce Message: Include the SMTP status code, the diagnostic code, and any descriptive text.
• Timestamp of the Attempt: Let support know when you tried sending the email so they can check logs.
• Recipient Address: Sometimes the issue is specific to a particular mailbox or domain.
• Your Domain Name: Indicate which domain you’re sending from if you manage multiple domains.

The more information you provide, the faster support can diagnose the root cause and help you resolve the issue.

#

Understanding email bounce errors is crucial to maintaining good deliverability. By identifying the specific SMTP code, you can quickly address issues like invalid addresses, full mailboxes, spam filtering, or authentication problems. With the right steps—verifying addresses, configuring email authentication, and checking for blacklists—you’ll significantly reduce bounce rates and ensure your emails arrive where they’re supposed to: the recipient’s inbox.

If you need further assistance, reach out to AEserver Support with details of your bounce messages, and we’ll be happy to help you troubleshoot and resolve any email delivery challenges.