If you get an error "550 5.7.1 <recipient>... Recipient not authorised - access denied" when sending an email from a VPOP3 user, then that means that the permissions for the sending user do not allow sending messages to that email address
Edit the User in the VPOP3 settings, go to the Permissions tab, and check the 'Allow sending of Internet mail' and 'Target whitelist' and 'Target Blacklist' options