When sending emails, in select instances, you can come across the "Could not complete sender verify" error message within your WHM mail queue. Or, if you've received a bounceback message, it's sometimes worded as "Could not verify sender", "Sender verify failed", or "550: failed to verify sender".
All of these errors essentially have the same meaning. One of the initial steps taken by the sending mail server is processing a list of checks, one of which includes verifying that the receiving email account exists on the destination mail server to help avoid spam.
If the sending mail server is unable to verify this, the email will fail delivery and generate one of the above error messages. However, oftentimes, the email account exists on the destination server, but other factors such as network connectivity issues or duplicate entries of the domain within the server can generate this error.
Below is a list of things you can review to ensure email sender verification can go through.
 "Mail Routing" for the destination domain may be misconfigured. The domain's mail routing should be set up where the mail servers are located. If the destination domain is not making use of external mail servers, then it should be set to "local".
 Your server is unable to communicate with the destination mail server. At times, the server's resolvers can be unresponsive, causing external connections to fail. A quick way to check your server's resolvers to verify connectivity is making use of the following internal cPanel script:
 A typo in the destination email account. Copying and pasting an email account from another source and forgetting a character, in the beginning, is a common mistake. You'll want to ensure the email account is spelled out correctly.
 Incorrect mail permissions. The destination server sometimes would have the email account on the server, but the mail server could be having issues accessing the email account due to incorrect permissions. This generally happens when a migration from one server to another occurs, and permissions are not retained. If you happen to have access to the destination mail server, you can run one of our internal cPanel scripts to check and correct permissions:
 The sender email account existing on the destination server. If the sending email account once used to exist on the destination server and was migrated, if the old mail contents still exist on the destination server, this can cause a false positive. To resolve this, you'll have to remove any email account files that belonged to the sender.
If the issue is still persistent after applying the above, then an additional review of the network interaction between the servers may be required.