Bounce-back messages can sometimes look confusing, but they usually contain useful information that explains why an email could not be delivered.
Below are some of the most common bounce-back messages and what they mean.
Mailbox not found / Invalid mailbox / User unknown / Not our customer
These messages usually mean the recipient's email address has been entered incorrectly or the mailbox no longer exists. Double-check the spelling of the email address and confirm the recipient's details before trying again.
These messages usually mean the recipient's email address has been entered incorrectly or the mailbox no longer exists. Double-check the spelling of the email address and confirm the recipient's details before trying again.
Mailbox unavailable
This often indicates a problem with the recipient's email account or server. Verify the email address is correct and try sending the message again later. If the issue continues, you may need to contact the recipient using another method.
This often indicates a problem with the recipient's email account or server. Verify the email address is correct and try sending the message again later. If the issue continues, you may need to contact the recipient using another method.
Mailbox full / Mail quota exceeded
The recipient's mailbox has run out of storage space and cannot accept new messages. Contact the recipient and ask them to free up space. Once they have done so, you will need to resend your message.
The recipient's mailbox has run out of storage space and cannot accept new messages. Contact the recipient and ask them to free up space. Once they have done so, you will need to resend your message.
Host unknown / Domain lookup failed
The part of the email address after the @ symbol cannot be found. This is commonly caused by a spelling mistake in the domain name or because the organisation has changed email providers or domains.
The part of the email address after the @ symbol cannot be found. This is commonly caused by a spelling mistake in the domain name or because the organisation has changed email providers or domains.
Unable to relay
This error can have several causes, including:
This error can have several causes, including:
- The sender's email address does not exist on the mail server.
- Email authentication is required before sending.
- The wrong outgoing (SMTP) server is being used.
No adequate servers / Connection timed out / Resources temporarily unavailable / Out of memory
These messages usually indicate a temporary issue with the recipient's mail server. Most mail servers will continue attempting delivery automatically for up to 48 hours before giving up.
These messages usually indicate a temporary issue with the recipient's mail server. Most mail servers will continue attempting delivery automatically for up to 48 hours before giving up.
Blocked / Listed in SpamCop, Spamhaus or similar services
The receiving mail server has rejected the message because the sending server's IP address has been listed on a spam blacklist. If you receive an error like this, please contact us and we will investigate.
The receiving mail server has rejected the message because the sending server's IP address has been listed on a spam blacklist. If you receive an error like this, please contact us and we will investigate.
Rejected as Spam
The recipient's spam filters have identified your message as potential spam. Review the bounce-back carefully, as it may provide additional information. Try sending a simpler test email first, then revise the original message if necessary.
The recipient's spam filters have identified your message as potential spam. Review the bounce-back carefully, as it may provide additional information. Try sending a simpler test email first, then revise the original message if necessary.
Security Warning: Fake Bounce Messages
Be cautious of bounce messages containing attachments.
Cybercriminals sometimes send fake bounce notifications containing malicious attachments. These emails usually encourage you to open an attachment for further details.
Never open unexpected attachments from bounce-back emails. Legitimate bounce messages do not require you to open attachments to understand why a message failed.
Cybercriminals sometimes send fake bounce notifications containing malicious attachments. These emails usually encourage you to open an attachment for further details.
Never open unexpected attachments from bounce-back emails. Legitimate bounce messages do not require you to open attachments to understand why a message failed.
You may also receive bounce messages relating to emails you never sent. This can happen when spam or malware uses a forged sender address that matches your own email address.
If Every Email Is Bouncing Back
If all outgoing email is failing, there may be a problem with your email client configuration. Check your outgoing (SMTP) server settings and account credentials. If the problem continues, please contact us and we will be happy to help.
