http://www.rfc-editor.org/rfc/rfc3598.txt
RFC 3598: Sieve Email Filtering -- Subaddress Extension, K. Murchison2003-09-01T23:00:00-00:00On email systems that allow for "subaddressing" or "detailed addressing" (e.g., "ken+sieve@example.org"), it is sometimes desirable to make comparisons against these sub-parts of addresses. This document defines an extension to the Sieve mail filtering language that allows users to compare against the user and detail parts of an address. [STANDARDS-TRACK]