Abstract The SPECIAL-USE capability of the IMAP protocol (RFC 6154) allows clients to identify special-use mailboxes@ e.g.@ where draft or sent messages should be put. This simplifies client configuration. 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 test for checking whether a special-use attribute is assigned for a particular mailbox or any mailbox@ and it adds the ability to file messages into a mailbox identified solely by a special-use attribute.