If you are seeing errors like this:
When someone is trying to send an email from outside of your server to an address inside your server, then it’s because the external server failed to pass the sender verification callout. This means that their mail server is not configured to respond to the verification checks that your server is doing.
To allow mail the mail through from the external server, you can add their server IP to the Sender Verification Bypass IP addresses. This is found in the Exim Configuration Editor (Basic Editor) in the WHM. Any IP addresses you add to this will no longer require sender verification, so their mail should go through without any problems