A server that advertises this extension MUST implement the

i;unicode-casemap comparator, as defined in [UCM]. It MAY implement

other comparators from the IANA registry established by [RFC4790].

See also Section 4.5 of this document.

A server that advertises this extension SHOULD use i;unicode-casemap

as the default comparator. (Note that i;unicode-casemap is the

default comparator for I18NLEVEL=1, but not necessarily the default

for I18NLEVEL=2.) The selection of the default comparator MAY be

adjustable by the server administrator, and MAY be sensitive to the

current user. Once the IMAP connection enters authenticated state,

the default comparator MUST remain static for the remainder of that

connection.