My Aim is to achieved configuration as CPanel provide as a MX backup server option. the background: Domain -> domain.com Main MX (priority 5) -> On-Promise Exchange server Secoundary/ Backup MX (priority 50) -> VPS (ISPConfig, postfix, etc) - server is hosting www for the domain.com the goal: All emails fro domain.com will be relayed to On-Promise Exchange server, even sent from the ISPConfig Config. The ISP server hosted the website with the form -> form sends the message to the firstname.lastname@example.org using local (ISPConfig server) smtp account email@example.com the problem: when i try to send email from firstname.lastname@example.org (IPSConfig server) to email@example.com (on-promise server) i have an message: --- Message not sent. Server replied: Requested action not taken: mailbox unavailable 550 5.1.1 <--->: Recipient address rejected: User unknown in virtual mailbox table --- setup: Relay Email Routing -> domain.com -> smtp, active, No MX lookup, exchange@ Relay recipients -> domain.com (whole domain) to send from the server web form via smtp: Email Domain -> domain.com (to create firstname.lastname@example.org account) email Mailbox -> email@example.com (mailbox witch I use to send emails) --- if I set up catch all the system works but forward all emails as a catch all email address. if i create local mailbox firstname.lastname@example.org its work as well. I need be able do forwarding emails independently - not based on the local mailboxes . How to do it ?