is invalid or DNS says does not exist



There was a SMTP communication problem with the recipient's
email server. Please contact your system administrator.

<server.DOMAIN.COM #5.5.0 smtp;501 <DOMAIN.co.uk> is invalid or
DNS says does not exist>


All external email address sent from OUTLOOK to internal EXHANGE are getting
rejected with the above error.

I added my ISP DNS IP to anti spam look up ... wondering they are blocking
because of too many lookups.

Scott


.