What does my email bounceback mean?

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When a mail server can’t 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, find its error code below to see what caused the issue and how to resolve it.

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.

Common Bouncebacks with Email

Bounceback ErrorCause of ErrorResolution
421 Server temporarily unavailable. Try again later. IB101The email queue is experiencing higher than normal email volume.Try again later.

Recipient Account Related Bounces

Bounceback ErrorCause of ErrorResolution
Account storage limitThe 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 existThe email the sender sent to does not exist.Verify that the recipient email was entered correctly.
Account disabledThe 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 timeoutThis 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

Bounceback ErrorCause of ErrorResolution
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 misspelledOr 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

Bounceback ErrorCause of ErrorResolution
535 Authentication not allowed on IBSMTP Servers. IB401Authentication 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. IB501The 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. IB502Email 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: IB506The 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. IB508The 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. IB108The 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 ErrorCause of ErrorResolution
552 This message has been rejected due to content judged to be spam by the internet community. IB212The 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. IB705The 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

Bounceback ErrorCause of ErrorResolution
452 This message contains too many recipients. Reduce the number of recipients and retry. IB605The 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. IB007This 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. IB110This 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. IB111This 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. IB112This 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. IB113This 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. IB504This 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. IB510This 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. IB607This IP address has reached the maximum allowed recipients for that hour.Try again later.

RBL Related Bounces

Bounceback ErrorCause of ErrorResolution
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). IB103This 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). IB104This 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). IB105This 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). IB106This 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 Microsoft 365

Bounceback ErrorCause of ErrorResolution
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 senderBad Outbound Sender – Messages sent from the Microsoft 365 email account may violate Microsoft’s anti-spam policy.Check the messages sent by the email address to see if they were sent intentionally, and then reach out to 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 the messages sent by the email address to see if they were sent intentionally, and then reach out to 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.

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