Fix rejected email with a bounce error

There are many reasons sent email can come back rejected with a bounce error. When this happens, find your error to understand what happened and how to solve the issue.

 Note: These examples use the domain coolexample.com and its email address holly@ — replace these with your own information.

Here are the most common error messages. Select a bounce type to see its errors and solutions:

Common email bounces

Bounce error	Cause of error	Solution
421 Server temporarily unavailable. Try again later. IB101	The email queue is experiencing higher than normal email volume.	Try again later.

Recipient bounces

Bounce error	Cause of error	Solution
Account storage limit	The recipient’s account has reached its storage limit and can’t receive email at this time.	The recipient needs to clear out their inbox or the receiving folder to make room for more email.
Account does not exist	The email address you sent to does not exist.	Verify the recipient email address was entered correctly.
Account disabled	The recipient account exists but its ability to receive mail was disabled.	Typically, these accounts remain permanently disabled, but you can try sending the email again later.
Delivery timeout	This could happen for several reasons.	Try again later.
This message is looping: it already has my Delivered-To line. (#5.4.6)	The recipient account is forwarding the message in a loop.	This is commonly caused by the receiver having two addresses that forward to each other. They need to correct their forwarding settings.

Invalid recipient bounces

Bounce error	Cause of error	Solution
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’re sending to a group, you must reach out to each address individually to determine who is triggering the error.

Sender bounces

Bounce error	Cause of error	Solution
535 Authentication not allowed on IBSMTP Servers. IB401	Authentication is not allowed on inbound mail. This happens when you have incorrect outgoing SMTP server settings in your email client (like Outlook or Gmail).	Set up your email client using the following SMTP server setting, smtpout.secureserver.net.
550 holly@coolexample.com Blank From: addresses are not allowed. 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. Provide a valid From. IB502	Email messages can’t 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. 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’re sending from a valid domain. Then verify that the domain has valid DNS records by checking your zone file. If the DNS isn’t valid, it must be fixed before you resend the email.
550 holly@coolexample.com Invalid SPF record. 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’re 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. ELITEWEB.Co manages reverse DNS when using our email services. We don’t support custom reverse DNS.

Content bounces

Bounce error	Cause of error	Solution
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.	Include an option to opt out in your messages. Then check your sending lists to ensure you’re 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, get a copy of the original message attempting to be sent, and then report the bounce back error.You should also make sure all links in the email go to valid destinations that are free of malware and phishing scams.
552 Virus infected message rejected. IB705	The email message containing a link, attachment, or pattern caught by our filters as a possible virus.	Include an option to opt out in your messages. Then check your sending lists to ensure you’re 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, get a copy of the original message attempting to be sent, and then report the bounce back error.You should also make sure all links in the email go to valid destinations that are free of malware and phishing scams.
552 Message rejected for spam or virus content	The email message contains a link, attachment, or pattern caught by our filters as spam.	This error is caused by sending outbound email where the recipients did not request to receive email from you. We don’t recommend sending outbound email from our email platforms. (Learn more about email marketing best practices.)You should also make sure all links in the email go to valid destinations that are free of malware and phishing scams.

Rate limiting bounces

Bounce error	Cause of error	Solution
452 This message contains too many recipients. Reduce the number of recipients and retry. IB605	The message has attempted to mail too many recipients.	Reduce the number of recipients and try again.
421 Connection refused, too many sessions from This IP. Lower the number of concurrent sessions. IB007	This IP address currently has too many sessions open.	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’re 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.
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.	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.	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.	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.	Try again later.
550 This message has exceeded the max number of messages per session. Open a new session and try again. IB510	This IP address has reached the maximum allowed messages for that session.	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.	Try again later.

Remote block list (RBL) bounces

Bounce error	Cause of error	Solution
554 RBL Reject.	This IP address was blocked from our internal RBL.	Follow the link provided in the bounce back 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).	Follow the link provided in the bounce back to check the reputation of the sending IP address in Cloudmark Sender Intelligence.
554 Connection refused. This IP is listed on the Spamhaus Block List (SBL). IB104	This IP address is listed on the Spamhaus Block List.	Check to see if the IP address is listed on the Spamhaus Block List.
554 Connection refused. This IP is listed on the Exploits Block List (XBL). IB105	This IP address 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 address is listed on the Spamhaus Policy Block List.	Check to see if the IP address is listed on the Spamhaus Policy Block List.

Microsoft 365 bounces

Bounce error	Cause of error	Solution
5.1.1 RESOLVER.ADR. ExRecipNot Found; not found	There are cached entries from the Nickname List, or you deleted your Nickname Cache File.	Try clearing cached entries from your Nickname list or deleting your Nickname Cache file. Learn more from Microsoft.
550 5.4.1 Recipient address rejected: Access denied. AS(201806281)	The address is not recognized by the recipient system, such as email arriving at an incorrect server due to a DNS issue, or the recipient address isn’t set up to receive email.	Make sure you entered the correct email address. Verify the recipients’ domain and MX records are properly configured for Microsoft 365. For more information, see this Microsoft article.
550 5.1.8 Access denied, bad outbound sender.	Bad Outbound Sender – Messages sent from the Microsoft 365 email account may violate Microsoft’s anti-spam policy.	Check if the messages were intentionally sent by the email address, and then contact customer support for help unblocking the address..
550 5.7.705 Service unavailable. Access denied, tenant has exceeded threshold.	Tenant Exceeded Threshold – Messages were sent from the Microsoft 365 organization that may violate Microsoft’s anti-spam policy.	Check if the messages were intentionally sent by the email address, and then contact customer support for help unblocking the address.
Your organization does not allow external forwarding.	By default, Microsoft doesn’t allow forwarding to external email addresses.	Edit your outbound spam filter policy.
550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP	You’re sending from known, low reputation IP addresses. You might also have a new or trial version of Microsoft 365.	Complete the appeal at sender.office.com or contact your Internet Service Provider (ISP) to do this.

Related steps

• Forward messages as attachments
• Forward messages as attachments in webmail

More info

• Workspace Email account limitations
• Microsoft 365 email account limitations