http://www.rfc-editor.org/rfc/rfc5255.txt
RFC 5255: Internet Message Access Protocol Internationalization, C. Newman, A. Gulbrandsen, A. Melnikov2008-06-01T23:00:00-00:00Internet Message Access Protocol (IMAP) version 4rev1 has basic support for non-ASCII characters in mailbox names and search substrings. It also supports non-ASCII message headers and content encoded as specified by Multipurpose Internet Mail Extensions (MIME). This specification defines a collection of IMAP extensions that improve international support including language negotiation for international error text, translations for namespace prefixes, and comparator negotiation for search, sort, and thread. [STANDARDS-TRACK]