Hello all,
If i want to add new MX in my domain name then what to do in new mx server.
Suppose my existing mail server is test.com whose mx record is below

test.com. 14400 IN MX 0 mail.test.com.
test.com. 14400 IN MX 5 mx-01.test23.net.

So, what need to change in test23.net server.

Your question isn't entirely clear. According to the fragment from the DNS configuration you posted, you want to create two mail servers, mail.test.com and mx-01.test23.net. Your DNS will report that these two servers will handle mail for test.com.

Per your priority settings, mail will first go to mail.test.com and if that is not available then it will go to the other one.

You will need to decide how you want this setup to work. I would suggest configuring test23.net a backup server. How you do this will depend on what mail system you are running. For example, if you are running postfix, there are a couple of settings that need to be changed in main.cf to tell it that it is a backup for the other server. It will then receive mail for that (other) domain and hold it for up to 60 days while periodically attempting to forward it to the primary server. As a backup mail server, it will do this rather than attempting to deliver it or relay it.