Supported

IDLE command - RFC 2177
QUOTA - RFC 2087
NAMESPACE - RFC 2342
CONDSTORE - RFC 4551
ENABLE - RFC 5161
LITERAL+ - RFC 2088
LOGINDISABLED - RFC 2595, RFC 3501
STARTTLS - RFC 2595, RFC 3501
UIDPLUS - RFC 4315
COMPRESS=DEFLATE - RFC 4978
XLIST - Supported in Apple iPhone, developed by Google and Apple to define standart folder names like drafts, sent, junk - especially important if they using international names.
MOVE http://tools.ietf.org/html/draft-krecicki-imap-move
ID - RFC 2971
EXTERAL SASL - RFC 4422
Special-Use Mailboxes - RFC 6154

Unsupported or partially implement

LOGIN-REFERRALS - RFC 2221, RFC 2193
MAILBOX-REFERRALS - RFC 2193
LANGUAGE, I18NLEVEL=1, I18NLEVEL=2 - RFC 5255
ACL, RIGHTS= - RFC 4314. Currently read-only. Write support (set/modify ACL) has patches for backend and code for frontend
BINARY - RFC 3516
CATENATE - RFC 4469
CONTEXT=SEARCH - RFC 5267
CONTEXT=SORT - RFC 5267
ESEARCH - RFC 4731
ESORT - RFC 5267
MULTIAPPEND - RFC 3502
QRESYNC - RFC 5162
SASL-IR - RFC 4959
SEARCHRES - RFC 5182
SORT - RFC 5256
THREAD - RFC 5256
UNSELECT - RFC 3691
URLAUTH - RFC 4467
WITHIN - RFC 5032
CHILDREN - RFC 3348
FILTERS - RFC 5466
LIST-EXTENDED - RFC 5258
CONVERT - RFC 5259
NOTIFY - RFC 5465
Return STATUS information in extended LIST - RFC 5819