Whomever your hosting provider for your site is the one you have to contact and ascertain if they are blocking outbound port 456 at the firewall level (or at any level).
Simply ask them that very statement above and they should be able to tell you yes or no. If yes, then you need to have them open that port in their firewall for outbound traffic or you will not be able to utilize the SMTP service on that port. If their response is no, then they need to research to find why the traffic is being blocked.
Is this a shared hosting provider or a VPS/dedicated server setup? If VPS/dedicated server it will be easy to do. If shared hosting then they should already have it opened but may not due to abuse(s) by other clients.