In [email protected], the Internet email domain is example.org—the part after the @. For ordinary Internet email, that domain is used in DNS lookups to find routing information for delivery; it is not necessarily the name of the server that ultimately receives the message.
Where the domain appears in an email address
RFC 5322 defines an address in the form local-part@domain. In [email protected], alex is the local-part and example.org is the domain. The local-part identifies a mailbox within the context of that domain; it does not by itself identify a globally unique mailbox.
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The ordinary domain form is an Internet domain name made of one or more labels, commonly separated by dots. RFC 5321 describes an SMTP domain name as a fully qualified name, rather than an informal local nickname or an unqualified alias, subject to the standard’s stated exceptions. See RFC 5322 §3.4 and RFC 5321 §2.3.5.
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What the domain does when mail is delivered
The domain tells the mail system which destination domain to look up. Under SMTP’s DNS resolution procedure, a sending system first looks for an MX record associated with the domain; DNS address records may also matter under the specified rules. An MX record points to a mail exchanger, so the address domain can be an alias or a label used for mail routing rather than the receiving server’s own hostname. The routing procedure is described in RFC 5321 §5.1.
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That distinction is why it is more accurate to call the domain the destination identifier used for mail delivery than to call it “the mail server.” A domain also does not have to host a website. Seeing a familiar domain in an address alone does not establish that a mailbox exists or that a message can be delivered.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.The technical exception: a domain-literal
Not every syntactically valid address uses a DNS-style name after the @. RFC 5322 also permits a domain-literal, which represents a literal Internet address in brackets rather than an ordinary domain name. The standard describes the domain portion as identifying “the point to which the mail is delivered” and distinguishes the dot-atom Internet domain-name form from the domain-literal form. See RFC 5322 §3.4.1. For everyday addresses, the dot-separated DNS domain is the form most people encounter.
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