That is really a big task to support via email. I'd suggest the following:
RedHat 8 and 9 are no longer supported by RedHat. No new updates or patches are being created. Only RedHat Enterprise is supported now and you have to pay for updates & support. That may be difficult from India.
If cost & support are limiting factors, then I'd upgrade to Fedora Core 1 which looks just like RedHat, RPMs and all. It is partially financed by RedHat and they seem to be incorporating Fedora features into their Enterprise edition. Fedora Core 2 is too unstable at the moment. My mail server runs on Fedora Core 1 quite well.
Creating a replacement mail server can be tricky. If users use POP to get mail from the server, then you'll have to recreate all their linux usernames on the new server. I don't know what you need to do if they use IMAP, I've never used it before, but it is the default email system used by Microsoft Exchange.
At the very least you'll have to make sure the files in /etc/mail on the new server matches that of the old one.
DNS & IP addresses will be another issue. You'll probably want to have the new mailserver have a different IP address from the old server. When everything is ready for the switch, then change DNS to point mail to the new server. This link should give you a guideline:
You'll also have to consider other applications running on the mail server. It may not just handle mail, but other things as well. Make sure you do a lot of research just to make sure.