You can easily change the mail server ip address in cpanel by using /etc/mailips file. The same as in PLESK you can also change the outgoing mail server ip address by using “/var/qmail/control/smtproutes” file.
first check the domain name which is hosted on the server by using following file
1) cat /var/qmail/control/rcpthosts
2) By default the “/var/qmail/control/smtproutes” file is not present on PLESK server, you need to create it.
3) Then add the new ip address in the following format.
4) Save the file and restart qmail
Replace the domain.com with your own domain name and the IP 111.222.333.444