How to setup sendmail V8 can send out to Internet Multidomain in one machine

I setup mail server can receive multidomain is "domain1.com" and "domain2.com" but I need , who are sent email to "user1@domain1.com" then user1 can reply mail to them by subject of Sender is "user1@domain1.com". If who are sent email to "user1@domain2.com" then user1 can reply mail to them by subject of Sender is "user1@domain2.com"
If you have any question sent about it to me.

Email : sophon.k@cdg.co.th

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Sophon.k