What does my email bounceback mean?

When a mail server cannot send or deliver an email, it generates a "bounceback" message that describes what happened, including the reason it failed.

If you receive a bounceback message, you can cross reference its error code with the information below to find out what caused the issue and how to resolve it.

These examples use the domain coolexample.com and its email address holly@ — you'll need to replace these with your own information.

Here are the most common bounceback message errors:

Use Ctrl + F to find keywords.

Common Bouncebacks with Email

Bounceback Error Cause of Error Resolution
421 Server temporarily unavailable. Try again later. IB101 The email queue is experiencing higher than normal email volume. Please try again later.

Recipient Account Related Bounces

Bounceback Error Cause of Error Resolution
Account storage limit The recipients account has reached it's storage limit and cannot receive email at this time. The recipient needs to clear out the receiving folder to make room for more email.
Account does not exist The email the sender sent to does not exist. Verify that the recipient email was entered correctly.
Account disabled The recipient account exists but its ability to receive mail has been disabled. Typically these accounts remain permanently disabled, but you can try sending the email again later.
Delivery timeout This could happen for possible reasons. Please try again later.

Invalid Recipient Bounces

Bounceback Error Cause of Error Resolution
550 Recipient not found. The recipient is not a valid email address. Remove the invalid recipient from your email.
451 Sorry, I wasn’t able to establish an SMTP connection. I’m not going to try again; this message has been in the queue too long.
  • The recipient's email address has been misspelled
  • Or the recipient's email provider has blocked sending from this address.
  • Check the email address for any errors.
  • If you are using AOL, check out how to unblock emails.
  • If you are sending to a group, you must reach out to each address individually, in order to determine who is triggering the error.

Sender Related Bounces

Bounceback Error Cause of Error Resolution
535 Authentication not allowed on IBSMTP Servers. IB401 Authentication is not allowed on inbound mail. Do not attempt to authenticate when sending mail to this server.
550 holly@coolexample.com Blank From: addresses are not allowed. Please provide a valid From. IB501 The email message "from" is blank. Add a valid "from" address and try to send the email again.
550 holly@coolexample.com IP addresses are not allowed as a From: Address. Please provide a valid From. IB502 Email messages cannot be sent "from" an IP Address. Add a valid "from" address and try to send the email again.
550 coolexample.com From: Domain is invalid. Please provide a valid From: IB506 The domain doesn't have valid MX Records, an IP address, or there were issues during the DNS lookup when the email was sent. Verify that you are sending from a valid domain. Then verify that the domain has valid DNS records by doing an external DNS lookup. If the DNS isn't valid, it must be fixed before you resend the email.
550 holly@coolexample.com Invalid SPF record. Please inspect your SPF settings, and try again. IB508 The sending email address's domain has an SPF record that does not authorize the sending email server to send email from the domain. Modify the SPF record to include the server you are trying to send from, or remove the SPF record from the domain.
421 Temporarily rejected. Reverse DNS for this IP failed. IB108 The IP address attempting to send mail does not have reverse DNS setup, or the DNS lookup failed. Verify the sending IP address has reverse DNS setup before resending the email.

Content Related Bounces

Bounceback Error Cause of Error Resolution
552 This message has been rejected due to content judged to be spam by the Internet community. IB212 The email message contains a link, attachment, or pattern caught by our filters as spam. Please include an option to opt out in your email messages. Then check your sending lists to ensure you are only sending to recipients who have selected to opt in to receiving your mail. If you feel this message has been flagged as spam erroneously, please be sure to obtain a copy of the original message attempting to be sent, and then contact support.
552 Virus infected message rejected. IB705 The email message containing a link, attachment, or pattern caught by our filters as a possible virus. Please include an option to opt out in your email messages. Then check your sending lists to ensure you are only sending to recipients who have selected to opt in to receiving your mail. If you feel this message has been flagged as spam erroneously, please be sure to obtain a copy of the original message attempting to be sent, and then contact support.

Rate Limiting Bounces

Bounceback Error Cause of Error Resolution
452 This message contains too many recipients Please reduce the number of recipients and retry. IB605 The message has attempted to mail too many recipients. Please reduce the number of recipients and try again.
421 Connection refused, too many sessions from This IP. Please lower the number of concurrent sessions. IB007 This IP address currently has too many sessions open. Please lower the number of open sessions and try again.
554 This IP has been temporarily blocked for attempting to send too many messages containing content judged to be spam by the Internet community. IB110 This IP address has attempted to send too many messages containing content judged to be spam and has been blocked for an hour. If you are not sending spam, you'll need to contact your Internet Service Provider (ISP) to see why your IP address is sending so many emails. Something in your system is causing the issue, and you'll need to troubleshoot. C3: To determine and confirm ownership of the IP address, you can do an IP address look up through DNS Stuff, senderbase.org or any other look-up tool.
554 This IP has been blocked for the day, for attempting to send too many messages containing content judged to be spam by the Internet community. IB111 This IP address has attempted to send too many messages containing content judged to be spam and has been blocked for the remainder of the day. Please check the content of your message and try again later.
554 This IP has been temporarily blocked for attempting to mail too many invalid recipients. IB112 This IP address has attempted to mail too many invalid recipients and has been blocked for an hour. Please check your recipient list and try again later.
554 This IP has been blocked for the day, for attempting to mail too many invalid recipients. IB113 This IP address has attempted to mail too many invalid recipients and has been blocked for the remainder of the day. Please check your recipient list and try again later.
550 This IP has sent too many messages this hour. IB504 This IP address has reached the maximum allowed messages for that hour. Please try again later.
550 This message has exceeded the max number of messages per session. Please open a new session and try again. IB510 This IP address has reached the maximum allowed messages for that session. Please try again later.
550 This IP has sent too many to too many recipients this hour. IB607 This IP address has reached the maximum allowed recipients for that hour. Please try again later.

RBL Related Bounces

Bounceback Error Cause of Error Resolution
554 RBL Reject. This IP address has been blocked from our internal RBL. Follow the link provided in the bounceback to submit a request to remove this IP address.
554 Connection refused. This IP has a poor reputation on Cloudmark Sender Intelligence (CSI). IB103 This IP address has a poor reputation on Cloudmark Sender Intelligence (CSI). Check Cloudmark Sender Intelligence to see the reputation of the sending IP.
554 Connection refused. This IP is listed on the Spamhaus Block List (SBL). IB104 This IP is listed on the Spamhaus Block List. Check to see if the IP is listed on the Spamhaus Block List.
554 Connection refused. This IP is listed on the Exploits Block List (XBL). IB105 This IP is listed on the Spamhaus Exploits Block List. Check to see if the IP is listed on the Spamhaus Exploits Block List.
554 Connection refused. This IP is listed on the Policy Block List (PBL). IB106 This IP is listed on the Spamhaus Policy Block List. Check to see if the IP is listed on the Spamhaus Policy Block List.

Common Bouncebacks with Hosted Exchange

Bounceback Error Cause of Error Resolution
IMCEAEX-_O=
CHOST_OU=EX
CHANGE+20AD
MINISTRATIVE
+20GROUP+20+28F
YDIBOHF23SPDLT
+29_CN=RECIPIENTS_
CN =coolexample@ex2.
secureserver.net #550
5.1.1 RESOLVER.ADR.
ExRecipNot
Found; not found ##
There are cached entries from the Nickname List or you deleted your Nickname Cache File. Please see Cleaning up Your Outlook Contacts Auto-completion Errors.
holly@coolexample.com
#550 5.1.1 RESOLVER.
ADR.ExRecipNotFound;
not found ##
The email arrived at the incorrect server, potentially caused by a DNS issue. Make sure you entered the correct email address. Verify the recipients' domain and MX records are properly configured for Hosted Exchange. For more information, see Updating DNS records.

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