http://www.rfc-editor.org/rfc/rfc6134.txt
RFC 6134: Sieve Extension: Externally Stored Lists, A. Melnikov, B. Leiba2011-07-01T23:00:00-00:00This document defines a Sieve extension for accessing externally stored lists -- lists whose members are stored externally to the script, such as using the Lightweight Directory Access Protocol (LDAP), the Application Configuration Access Protocol (ACAP), vCard Extensions to WebDAV (CardDAV), or relational databases. [STANDARDS-TRACK]