FastMail The OBJECTID capability of the IMAP protocol (I-D.ietf-extra-imap- objectid) allows clients to identify mailboxes by a unique identifier which survives rename. In contrast, the Sieve mail filtering language (RFC 5228) currently has no such capability. This memo defines a Sieve extension that fills this gap: it adds a method for specifying the unique identifier of a mailbox as a target for fileinto rules, and a method for testing the existence of a mailbox by its unique identifier.