Multiple companies, 1 Exchange server - Correct MX and A Record settings, and RDNS

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 = 111.111.111.111  ???
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?
egandersAsked:
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PapertripCommented:
You only need an A record if you want firmb.com and/or any subdomains to resolve to an IP.

If you setup the MX record for firmb.com to point to mail.firma.com, then no additional A records need to be added.  If on the other hand you want the MX record to be mail.firmb.com, then you would also need an A record for mail.firmb.com, and it would point to the same IP that mail.firma.com does.

Reverse DNS is for the IP, so unless you are adding additional public IP's you shouldn't need to change that provided it's currently setup properly (the FQDN of your mail server matches the PTR record for the IP).

You should also create SPF records for firmb.com and firmc.com that include the IP(s) of your sending server.
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PapertripCommented:
Oh I should add in regards to my Reverse DNS comment -- this only really matters for the IP's of your sending servers, it is not required for receiving-only mail servers.
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egandersAuthor Commented:
1. I think you're saying that unless mail.firmB.com and mail.firmC.com use different IP's than mail.firmA.com, they don't need an "A records".   So would it be accurate to state that we do not want an "A record" for B and C in this scenario, else it will cause problems?

2. For a single Exchange server, and to make it easy, all mail for all three domains will go in and out of the same IP address.  So the Reverse DNS would only need to be setup for mail.FirmA.com?  Or does it need to be setup (with the ISP) for mail.FirmB.com and mail.FirmC.com too?

And does any of this have any impact on FcRDNS, or are there other settings I need for that?
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PapertripCommented:
1. I think you're saying that unless mail.firmB.com and mail.firmC.com use different IP's than mail.firmA.com, they don't need an "A records".   So would it be accurate to state that we do not want an "A record" for B and C in this scenario, else it will cause problems?
I mean they don't need A records if you don't need for example mail.firmB.com to resolve.  If you set the MX of firmB.com to mail.firmB.com, then mail.firmB.com would need an A record.  If you set the MX to mail.firmA.com, then you don't need to add any A records to the firmB.com zone.  Either way is fine and is up to you which you choose, personally I would do the former.

2. For a single Exchange server, and to make it easy, all mail for all three domains will go in and out of the same IP address.  So the Reverse DNS would only need to be setup for mail.FirmA.com?  Or does it need to be setup (with the ISP) for mail.FirmB.com and mail.FirmC.com too?
You can only have 1 PTR record per IP.  The PTR should be whatever you configured your Exchange server to be named, like mail.firmA.com.

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