Undeliverable or Bounce Error Messages When Sending Emails to Kayako Classic

Overview

You might get a report from end-users about getting "undeliverable" or bounce error messages (mailer-daemon) when trying to send emails to your mailbox or email queue on Kayako Classic. 

Solution

The most common cause of this issue is that the email address of the sender may have been blocked by your email server. An immediate solution that you can try is to whitelist or unblock the email address of the sender from your mail server. Your mail administrator should be able to help you in unblocking or whitelisting the email address.

Additionally, depending on the error code or message, you can take specific actions based on the recommendation such as the information found at the Mimecast SMTP Error Codes.