Relaying through MX Servers

It is important to realize that mail is not sent directly to the host whose name follows the@ in the email address. Instead, that host name is first looked up using DNS to find the names of its MX (mail exchange) servers. To illustrate, consider the email address bob@example.com. Before mail is sent to example.com, that host’s MX records are looked up:

example.com.   MX 10 mail.example.com.
example.com.   MX 20 bob.fallback.host.

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