The MX Algorithm

That's the basic idea behind MX records and mail exchangers, but there are a few more wrinkles you should know about. To avoid routing loops, mailers need to use a slightly more complicated algorithm than what we've described when they determine where to send mail.[1]

[1] This algorithm is based on RFC 974, which describes how Internet mail routing works.

Imagine what would happen if mailers didn't check for routing loops. Let's say you send mail from your workstation to [email protected], raving (or raging) about the quality of this book. Unfortunately, ora.oreilly.com is down at the moment. No problem! Recall oreilly.com's MX records:

oreilly.com. IN MX 0 ora.oreill " "

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