In alex@example.org, example.org is the email domain: the part after the @. It identifies the destination domain for the message, and mail systems use DNS records—ordinarily an MX record—to determine where to route it.
Where the domain appears in an email address
An email address is commonly written as local-part@domain. In alex@example.org, alex is the local-part and example.org is the domain. The local-part identifies a mailbox within the domain’s context; the domain identifies the delivery destination.
RFC 5322 §3.4.1 describes the domain portion as identifying “the point to which the mail is delivered.” This is a logical destination, not necessarily the name of the server that ultimately receives the message.
What an email domain does
In the usual form, the domain is an Internet domain name made up of one or more labels, commonly separated by dots. For example, example.org has the labels example and org. RFC 5321 §2.3.5 describes an SMTP domain as a fully qualified domain name, rather than an unqualified local nickname.
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When sending mail, an SMTP client looks up the destination domain in DNS and first attempts to find an MX record, as specified in RFC 5321 §5.1. The MX record supplies mail-routing information. A domain can be an alias or a name used for mail-exchanger records, so it does not have to be the hostname of the receiving server.
Is everything after @ the domain?
For the common address form, yes: the domain is the component after @. There is a standards-defined exception: RFC 5322 also permits a domain-literal, which represents a literal Internet address in brackets rather than a DNS-style domain name. This is a distinct, less ordinary form; it should not be confused with the usual domain-name syntax.
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What the domain does not tell you
- A domain is not necessarily the mail provider’s brand or the final receiving server’s hostname.
- Having an email domain does not mean it has a website; web hosting and mail routing are separate uses of domain and DNS infrastructure.
- The visible domain by itself does not prove that a particular mailbox exists or that a message can be delivered.
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