edit msg-<instance>/imta/config/imta.cnf
find the tcp_local channnel section, add a keyword of 'daemon <hostname>' to send all mail to that host.
You can also use an MX record for the value of hostname. So you could have load balance and fault tolerance.
Something like
! tcp_local
tcp_local smtp mx daemon send.mail.to.me single_sys remotehost inner switchchannel identnonelimited \
subdirs 20 maxjobs 7 pool SMTP_POOL maytlsserver maysaslserver saslswitchchannel tcp_auth
tcp-daemon
Don't forget to do
msg-<instance>/imsimta cnbuild
msg-<instance>/imsimta restart dispatcher
Regards,
Chad