Sending a message to a remote server can result in a bounceback with the error code "550 Access denied".
The error is typically an indication that the remote server is blocking messages from the sending IP/server.
The cPanel technical support team has no access to remote servers' blocklists or blacklists to see why the block was implemented or remove it on your behalf.
It would be necessary to review the bounceback message for any information on the block and what is needed to remove it. If there isn't any information provided, it would be necessary to contact the remote server's support or system administrator regarding this block.