Understanding email bounce-back messages

There are many different types of bounce-back messages which, at first glance, can seem very confusing. They key to understanding these bounce-back messages lies in being able to extract the key information and using this to understand why you are receiving the bounce-back.

Below are some of the most common types of bounce-backs received and what they mean.


Error Message:

Mailbox not found

Invalid mailbox

User unknown

Not our customer


The most common reason for any of the above error messages is that the email address was typed incorrectly or the mailbox that you are trying to reach no longer exists.


Error Message:

Mailbox unavailable

Mailbox not found


If you receive any of the above errors, chances are that there is a problem with the recipient’s email account. Check that you have the spelling of the email correct and wait a while before trying again. If the problem persists, you may need to contact the recipient by other methods.


Error Message:

Mailbox full

Mail quota exceeded


An error message containing the above means that the recipient has too many emails in their account and the mail server will not accept any more email until more space is made. Try contacting the recipient in another way to let them know. You must send the message again once they have created more space. This can also be a sign of an abandoned or unused account. Double check with the intended recipient that this is the best email address through another method of contact.


Error Message:

Host unknown

Domain look up failed


The above errors mean that part of the email address after the @ does not exist. This can be due to incorrect spelling or the company or ISP changing their name. Check that the spelling is correct first of all and then contact the recipient through another method to check that the information you have is accurate and up to date.


Error Message:

Unable to relay


This is a very general term, which covers a range of possible causes, such as:

-          The “from” address may not match any accounts on the mail server.

-          The mail server may require authentication before the email is sent

-          The mail server may require the email to come from a different outgoing server

Any of the above are conditions set by the recipient’s mail server. Ensure that the information that you have is correct before sending. If necessary, contact us and we can narrow down the fault and resolve the issue.


Error Message:

No adequate servers

Connection timed out

Resources temporarily unavailable

Out of memory


The above error messages relate to temporary problems with a mail server, which usually resolve themselves over time. The bounce back will usually advise if the mail will continue to try and send or if you should try to resend at a later time. The server will usually try to resend over a 48 hour period. You will receive a notification after this if delivery has failed.


Error Message:

If your email bounces back with a message containing the words “blocked” or “listed in” and there are references to sites like “spamcop”, “dynablock” or “spamhaus” then your email has been blocked by the recipients mail server. This is usually because the IP address of your own mail server is on a blacklist, as someone has been reported as sending large amounts of spam from the same mail server. If you receive a bounce back message like this, contact us immediately and we will investigate.


Error Message:

If you receive a bounce back message containing the words “rejected” and “spam” then your message has been blocked by the recipient’s content filters, as it looks too much like spam. Read the bounce back message thoroughly, as it sometimes contains more information as to why the message looks like spam. Try sending a simpler message to the recipient to test that they can receive messages from you. If this is successful, re-word your email to make it look more professional and less like spam.



“Bounces” that contain viruses

Spammers try to infect your computer by sending fake bounce messages which contain an attachment and advise you to open for full details. NEVER open the attachment, as this will more than likely be a virus. All legitimate bounce messages will be contained in the email and have an attachment. Delete any suspicious looking bounce backs straight away.


You may also be getting bounce messages for email you didn’t send. There’s another class of virus that spoofs or fakes the “from” address on emails, and as a result you could be getting bounce messages that have nothing to do with you.


Everything bouncing?

If everything you send bounces, then you probably have a different problem. Chances are your email client is misconfigured. Double check your outgoing or SMTP server settings and if the problem continues, get in touch and we’ll help resolve the issue. 

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