This error is caused because the IP of the server that sent the mail does not have a reverse lookup configured so that the IP can be translated to a name or, reverse resolution does not match the hostname. This reverse resolution must be configured by your ISP.

This error indicates that the system failed to find the specified user in the domain. You should make sure that the email address you have is correct.

This error indicates that the server that sent the mail server is not listed as authorized to send mail for that domain according to the domain SPF record.

If it is a legitimate email sent from one of your servers, you should check the SPF record and correct it so that it can validate and deliver mail.