I am trying to troubleshoot some problems sending mail from one of our WHM/cPanel 11 servers to another WHM/cPanel 11 server. Both servers have the "** Verify the existence of email senders." option enabled. I can successfully send an email from email@example.com to firstname.lastname@example.org, but i can not send an email from email@example.com to firstname.lastname@example.org, i get the sender verify failed response. First we thought it was because of an incorrect rDNS, but we fixed that, and it still fails. I noticed there are two different options in the exim configuration editor: "Use callouts to verify the existence of email senders. Basiclly, exim will connect to the mail exchanger for a given address to make sure it exists before accepting mail from it." which is disabled on both servers. "** Verify the existence of email senders." which is enabled on both servers. What is the difference and which one should I be using? Any ideas why sender verification would be failing? An interesting note, if i send from email@example.com to firstname.lastname@example.org, it sends successfully, host.server2.com is the FQDN hostname of server2.