First firm owns the existing Exchange server that has been setup and working for years. Two additional firms are now moving into the office and will be sharing the Exchange Server.. Each firm has their own domain name. In order they are FirmA.com, FirmB.com and FirmC.com. I've added the additional domains to the Exchange server. I'm not sure which DNS registrar settings should be used for the 2 additional firms.
The original FirmA.com has already been setup and working, and I've added the two additional domains to the Exchange server. The two new domains are registered with different domain registrars.
For FirmB.com and FirmC.com, do I setup an "A Record" for each of them at their registrars?
Or should I not even have an "A Record" What names and IP addresses should be referenced?
For Example: should the "A Record" = mail.FirmB.com = 18.104.22.168 ???
And what about an MX Record for Firms B and C? What should that say?
"MX Record" = mail.FirmA.com ???
And then for a Reverse DNS for FirmB and FirmC, Do they even have a Reverse DNS? Does it point to the FirmA IP, or what? Will that work with FcRDNS?