Mail server change

I am new to R/t and am trying to administer a R/T system deployed by someone
else.the ticketing system runs on Linux.
I am given the task of
1.keeping the existing domain eg;
But changing all email to go to a email server that is in a difference
domain called

so the server where R/t sits will be, but all emails should
get sent to a mailserver/people who have an id beloning to

Can anyone suggest what I need to do. I need to get this done by tomorrow
before the mail server switch to permanently.


has domains:
do I need to change that. I need to change only stuff related to email and
keep the domain same.