Many mail servers deliberately stall the display of the SMTP greeting by 30s or more. Any compliant mail system will wait until the greeting banner is displayed. The RFC standard says that the connecting server should wait up to 300 seconds.
When the cpanel server is trying to perform the sender verify callout and the greeting is stalled by 30s or more, it's at this point that the cpanel server reaches timeout. The cpanel server incorrectly assumes that the server it is connecting to isnt listening, when in actual fact, it is going through the stall period.
Is there anything in the exim configuration that can be changed to make this work ?