Often, if an email delivery fails, the original email is returned to its sender with an error explaining what caused the issue. These returned emails are referred to as bounce-back emails.

We prepared a list of common bounce back errors to help identify why your email might not be delivered to its intended recipient(s):

Mailbox not found
Invalid mailbox
User unknown
Mailbox unavailable
Mailbox not found

The most common reason for all of the above errors is that the email address was typed incorrectly or the mailbox that you are trying to reach no longer exists. You should also check that you have the spelling of the email correct.

Mailbox full
Mail quota exceeded

These error messages are showing that the recipient’s mailbox is full and the server will not accept any more messages until the user make more space available.

Connection timed out
Resources temporarily unavailable

The above errors relate to temporary problems with a mail server. The server will usually try to resend your message over a 48 hour period. After that timeframe, you should receive a notification in case the delivery has failed.

Unroutable address

This type of error can be due to a variety of reasons. The recipient’s email address might be misspelled. You also might have exceeded the maximum number of e-mails you are allowed to send per hour, in which case you should allow some time and then resend the mail. Your mail server might also be unable to determine the correct route to send your email message because of a DNS configuration error with the recipient domain’s MX records. This can be addressed by the owner of the recipient address.

Could not complete sender verify

This is a common error which happens when the username that you are authenticating your emails with is not correct. Make sure that you configure your email application to use the full email address as username instead and this issue should be resolved.

In addition to that, the error can occur if the recipients’ mail server require valid SPF or DKIM records to be set up before they start accepting mail from your domain. More information on how to set up SPF can be found on our knowledgebase.

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