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Exchange Server uses both internal and external DNS -- internal DNS for name resolution within your LAN, and external DNS to resolve names for outside e-mail addresses (required by SMTP). External DNS entries are usually set up by your Internet Service Provider, but internal DNS entries must be set up manually.
Some administrators seem to believe that internal DNS names don't have to be as strictly formatted as external DNS names, because people from the outside world don't need to resolve them in their clients. This may be true for the most part, but if your syntax and formatting don't conform with the RFCs for DNS, you could be asking for trouble when it comes to Internet- and third-party applications.
One mistake I have seen more than once is using an underscore (_) in an internal DNS name, such as server_1.local (just to choose an arbitrary example). The underscore is not considered a legal character in a domain name, so this breaks a primary DNS-naming rule.
The most basic rule of thumb for creating internal DNS names is that they should consist of alpha-numerics, a hyphen -- and nothing else. But even this standard needs to be subjected to scrutiny, because another less commonly known rule is that you shouldn't have a hyphen at the beginning or end of a domain entry. For instance, this-.local would not be a valid name, but this-server.local is fine. RFC 1035 has the most complete overview of what's valid for domain names and in what form.
This all impacts Exchange Server for a variety of reasons, not the least of which being that an Exchange server that can't be resolved correctly, or has issues resolving other servers, is going to malfunction. For instance, you could probably send to another domain without issue, but mail between local domains could fail.
In theory, it's possible to add disallowed characters to a domain name by using escape sequences. But there's always the chance of a compatibility problem with a third-party application that doesn't understand how to deal with escapes.
Technically, this is a problem with the program and not DNS itself, but it's generally easier to choose good internal DNS naming than it is to have third-party software rewritten.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.
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Related information from SearchExchange.com:
Tip: Common Exchange Server mail delivery issues
Tutorial: A primer on DNS and MX records
Reference Center: Exchange and DNS tips and resources
