If I understand you correctly you need to determine routing of some outbound email based on the recipient. This is not a core functionality of CiviCRM.
Outbound, by extension
CiviCRM allows extensions to implement hooks, and I think for your needs the appropriate hook would be
This hook is called when CiviCRM prepares an email driver class to handle outbound message delivery.
The documentation linked above for this hook gives example code to deliver emails to a logfile instead of via SMTP. Creating an extension which implements this hook and routes email to your CEP server when required would hopefully deliver the outcome you need. If open-sourced, this would be available to other organisations with the same requirements, which is a great outcome.
Outbound, in MTA
Some mail servers (MTA) allow you to customise routing based on destination. If you control your hosting environment, you might be able to instruct an MTA to route emails for some recipients to be delivered via one channel (SMTP to a specific CEP service, or add headers to CC/sign per requirements), and others via default SMTP routing. An MTA could query for the correct behaviour based on destination email against CiviCRM's HTTP API.
If you need to handle this on the inbound channel (when your organisation receives email), it's possible to configure multiple mailboxes to check in CiviCRM. This is done at Administer > CiviMail > Mail accounts and has documentation in CiviMail incoming mail setup and CiviCRM inbound mail documentation.