What does my email bounce mean?
When a sent message can't be delivered, it bounces or comes back with an error message (also called a bounceback) that describes what happened, including the reason delivery failed. If your message was undeliverable, find its error message below to see what caused the issue and how to resolve it.
The bounce error will show in the received message. This is how an error might look in Outlook, though your error message could look different depending on the email provider rejecting the message.
Use Ctrl+F (Windows) or Cmd+F (macOS) to find keywords.
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:
- Recipient Account Related Bounces
- Invalid Recipient Bounces
- Sender Related Bounces
- Content Related Bounces
- Rate Limiting Bounces
- RBL Related Bounces
- Common Microsoft 365 Bounces
Common Email Bounces
| Bounce Error | Cause of Error | Resolution |
|---|---|---|
| 421 Server temporarily unavailable. Try again later. IB101 | The email queue is experiencing higher than normal email volume. | Try again later. |
Recipient Account Related Bounces
| Bounce Error | Cause of Error | Resolution |
|---|---|---|
| Account storage limit | The recipients account has reached its storage limit and can't 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 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 | 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're sending to a group, you must reach out to each address individually in order to determine who is triggering the error. |
Sender Related Bounces
| Bounce 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. 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 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. 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. |
Content Related Bounces
| Bounce 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. | Include an option to opt out in your email 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, be sure to obtain a copy of the original message attempting to be sent, and then report the bounceback error. |
| 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 email 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, be sure to obtain a copy of the original message attempting to be sent, and then report the bounceback error. |
Rate Limiting Bounces
| Bounce Error | Cause of Error | Resolution |
|---|---|---|
| 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.
C3: To determine and confirm ownership of the IP address, you can do an IP address look up through DNSstuff or IP registries such as arin.net(USA and Canada), ripe.net (Europe), lacnic.net (Central and South America), afrinic.net (Africa), and apnic.net (Asia/Pacific) |
| 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. |
RBL Related Bounces
| Bounce 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 Microsoft 365 Bounces
| Bounce 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. | Try clearing cached entries from your Nickname list or deleting your Nickname Cache file. |
| 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 Check and manage my MX records. |
| 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 submit an Unblock Email request. |
| 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 submit an Unblock Email request. |
| Your organization does not allow external forwarding. | By default, Microsoft doesn't allow forwarding to external email addresses. | Edit your outbound spam filter policy. |
