[exim] Error 451 Could not complete sender verify callout

We are running Exim 4.63 on a dedicated server thru HG running Redhat 9 and cPanel 10.9.0-R118.
Several days ago mail started failing to certain MTA's due to the error 451 Could not complete
sender verify callout.



A check at dnstuff.com determined that our server was failing to respond to these tests. Tech
Support has been totally useless and I am desperately seeking some advice. What can we look at
to fix this problem??



Sincerely,

WendyH

SonicFog