This sendmail(1M) message usually indicates that the local host is trying to send mail to itself.
Check the value of the $j macro in the /etc/mail/sendmail.cf file to ensure that this value is a fully qualified domain name.
When the sending system provides its host name to the receiving system (in the SMTP HELO command), the receiving system compares its name to the sender's name. If these are the same, the receiving system issues this error message and closes the connection. The name provided in the HELO command is the value of the $j macro.