localpartdomain

Localpartdomain is an informal term used in some technical writings to refer to the mailbox identifier formed by the combination of the local part and its domain in an email address. It is not a formal RFC term. In this sense, localpartdomain denotes the complete address that points to a specific mailbox on a particular mail system, for example [email protected], where "alice" is the local part and "example.com" is the domain.

The local part is the portion before the at-sign and is used by the receiving mail system to determine the destination mailbox within the hosting domain. The domain identifies the mail system responsible for that mailbox. How the local part is interpreted can vary by server; some systems treat it as case-sensitive, others as case-insensitive, and many enforce additional policy restrictions on allowed characters or formats.

Character rules and limits for local parts are defined by email addressing standards and server policies. The domain part must be a valid domain name (with support for internationalized domain names via IDNA in modern implementations). Local parts can be complex, including quoted strings or escaped characters in some cases, but practical implementations often restrict characters to a more conservative set to ensure interoperability. Typical, though not universal, limits include a local part up to 64 characters and an overall address length that commonly remains within practical bounds around 254 to 256 characters.

Usage and applications of the term localpartdomain appear mainly in documentation, data models, and software design discussions where distinguishing between the local-part and domain is important for routing, authentication, or policy decisions. See also: email address, RFC 5321, SMTP, IDNA.