As mails flow from sender to recipient through the Email Security.Cloud system, there are two separate places that an SMTP connection can occur. By extension therefore, there are also two points of contact which can potentially cause a server error.
The following illustration shows how and when bounce-backs are generated
A quick and easy way for an administrator to establish where a bounce-back has been generated is to review the postmaster email address of the bounce email.
If the sending server encounters an error when connecting to Email Security.Cloud, the sending server will issue the bounce-back.
However, if the sending address of the bounce-back is from "[email protected]" , then the receiving server issued an error when Email Security.Cloud attempted to deliver the message to the intended domain.
The bounce-back will always be generated by the last MTA in possession of the email before being accepted by the next hop.