kumomta/docs/userguide/policy/routing.md

Configuring Message Routing

If you need to "smarthost" or route messages through another server, you can do this to put it in a queue whose domain is imap.server.hostname and it will resolve the MX/A -> ip address as normal.

msg:set_meta('queue', 'imap.server.hostname') 

This should me located in a 'smtp_server_message_received' function like this:

kumo.on('smtp_server_message_received', function(msg)
    ...
    msg:set_meta('queue', 'my.smarthost.com')

end)