The following technique can be used to see if Exim can verify a sender.
You may also be interested in the following resource for more context:
1. Login to your cPanel server via SSH or Terminal as the root user
2. Execute the following command with with the email address that would be sending a message to the cPanel server that you are logged into:
exim -bvs firstname.lastname@example.org
If this does not provide enough detail, you may add the -d flag to the command to provide a great deal of extra information:
exim -d -bvs email@example.com
This is documented here:
Here are some key excerpts:
Verification differs from address testing (the -bt option) in that routers that have no_verify set are skipped, and if the address is accepted by a router that has fail_verify set, verification fails. The address is verified as a recipient if -bv is used; to test verification for a sender address, -bvs should be used.
This option acts like -bv, but verifies the address as a sender rather than a recipient address. This affects any rewriting and qualification that might happen.