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Reverse DNS - 1 email server, 2 domain names

Posted on 2009-05-12
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I have an SBS 2003 server.  We use this exchange server to send and receive email for 2 different domains. xxx.com & yyy.com.   The DNS service I use has 1 MX record for each FQDN pointing to the exchange server.  Mail.xxx.com = xxx.168.1.2 and  mail.yyy.com also points to the exchange server at xxx.168.1.2.  I had AT&T (our DSL provider) create a reverse DNS entry for me@.xxx,com,  Not problem sending and receiving using this domain name, however, me@yyy.com email gets rejected sometimes because of no reverse DNS entry for mail.yyy.com.  AT&T tells me that only 1 FQDN can resolve to 1 IP address.  A strict 1 for 1 lookup.  How do I work around this problem?

Thanks for your help
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Question by:JackAitken
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kyodai earned 2000 total points
ID: 24370562
AT&T is unfortunately right here, a reverse lookup can only return one match for an IP Address, not several. The only RFC conform solution here is having a seperate MX record and IP for each domain.
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by:Mestha
ID: 24370601
There is no problem here.

Reverse DNS doesn't look at the email address of the message. All the reverse DNS lookup is doing is ensuring that the server resolves correctly, to a valid address. Some more selective servers will also look at the SMTP banner which should also match.
The reverse DNS host can be a completely different domain - mine for example is on a .net domain, whereas my email address is co.uk, I have no problems sending email because the DNS is setup correctly.

There are simply not enough IP addresses in the world for each domain to have its own reverse DNS address.

Simon.
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Author Comment

by:JackAitken
ID: 24371028
I do have 2 MX records.  Mail.xxx.com & mail.yyy.com.  They point to the same ip and same exchange server.  
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by:Rajith Enchiparambil
ID: 24372573
You can have any number of MX records pointing to the same ip. But, you only need to have ONE valid ptr record. Make sure that your banner matches as well.

Put your domain details in http://www.mxtoolbox.com/diagnostic.aspx and see what banner, ptr record is returned.

Rajith.
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Expert Comment

by:Mestha
ID: 24372817
I usually use the same MX record host for every domain hosted by a server as well. It means one set of DNS records to maintain. You can have an MX record host in a different domain. Again to use my own domains - all of my DNS is in a .net domain, even though my email domain is a co.uk.

As long as the MX record address resolves, it will be fine for inbound email.

Simon.
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