DNS Electronic Mail Support and Mail Exchange (MX) Resource Records
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Most savvy users of the Internet know that the Domain Name System exists, and usually associate it with the most common Internet applications. Of these applications, the big kahuna is of course the World Wide Web. It's probably the case that the majority of DNS name resolution requests are spawned as a result of Web server domain names being typed into browsers billions of times a day, as well as requests for named pages generated by both user mouse clicks and Web-based applications.
Of course, DNS is not tied specifically to any one application. We can specify names in any place where an IP address would go. For example, you can use a DNS name instead of an address for an FTP client, or even for a troubleshooting utility like traceroute or ping. The resolver will in each case take care of translating the name for you.
There's one application that has always used DNS, but it's one that doesn't usually spring to mind when you think about DNS: electronic mail. Electronic mail is in fact more reliant on DNS than just about any other TCP/IP application. Consider that while you may sometimes type in an IP address for a command like traceroute, or even type it into a browser, you probably have never sent anyone mail by entering email@example.com into your e-mail client. At least I never have in over a dozen years being online. You instead type firstname.lastname@example.org, and DNS takes care of figuring out where electronic mail for XYZ Industries is to go.
Name resolution for electronic mail addresses is different from other applications in DNS, for three reasons that I describe in the overview topic on TCP/IP e-mail addressing and address resolution. To summarize: first, we may not want electronic mail to go to the exact machine specified by the address; second, we need to be able to change server names without changing everyone's e-mail address; and third, we need to be able to support multiple servers for handling mail.
For example, XYZ Industries might want to use a dedicated mail server called mail.xyzindustries.com to handle incoming mail, but actually construct all of its e-mail addresses to use @xyzindustries.com. This makes addresses shorter, and allows the server's name to be changed without affecting user addresses. If the company wishes, it might decide to use two servers, mail1.xyzindustries.com and mail2.xyzindustries.com, for redundancy, and again have just @xyzindustries.com for addresses.
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