DNS, rDNS, SPAM, and Multiple Domains

We are in the process of buying out a company and along with that, come their email.  We learned the hard lesson about DNS, RDNS, and domain name resolution for spam purposes.

What we need to know is, how to tie multiple domain names to 1 rDNS record.  I would imagine that this is not possible, but on the other hand, I see where large hosting companies would have to do this without having to consume large blocks of IP addresses....
CandSNetworkingAsked:
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Simon Butler (Sembee)ConsultantCommented:
The PTR record (reverse DNS) does NOT have to match anything connected to the domain the email is coming from.
Therefore as long as the three records all match (A record, PTR and the SMTP banner) then you should be fine with regards to email delivery.

There are not enough IP addresses in the world for every domain to have its own for PTR purposes. I am aware of one server in the USA that hosts email for over 250,000 domains, single host name.

The common trick when it comes to hosting multiple domains is to have a single generic domain that is used for the MX record for all domains.

Simon.
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CandSNetworkingAuthor Commented:
So, you're saying that the MX of company 2 needs to simply be the MX of company 1?

So, if my first company has published mail.company1.com as my MX, and we buy Company2, we just set the MX record for company2 to the same setting as company 1?

Are you aware of a simple document that would outline how to achieve this?

Thanks Simon!

Sky
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Simon Butler (Sembee)ConsultantCommented:
Yes, that is all there is to it.

There is no connection between the MX record host and the domain that you are sending to.
All an MX record does is tell you that an email for example.com goes to host.example.net,

Nothing else has to be done - just make the change of MX record host in the control panel for wherever the domain is hosted.

Simon.
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CandSNetworkingAuthor Commented:
Thanks Simon.  We're moving forward now, thanks to your help.
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