lavabit.com: IMAP4rev1 STARTTLS LITERAL+
lua-ml: imap4rev1
fastmail.fm: IMAP4rev1 LITERAL+ ID ENABLE COMPRESS=DEFLATE RIGHTS=kxte QUOTA MAILBOX-REFERRALS NAMESPACE UIDPLUS NO_ATOMIC_RENAME UNSELECT CHILDREN MULTIAPPEND BINARY CATENATE CONDSTORE ESEARCH SORT SORT=MODSEQ SORT=DISPLAY SORT=UID THREAD=ORDEREDSUBJECT THREAD=REFERENCES ANNOTATEMORE ANNOTATE-EXPERIMENT-1 METADATA LIST-EXTENDED LIST-STATUS WITHIN QRESYNC SCAN XLIST XMOVE SPECIAL-USE CREATE-SPECIAL-USE DIGEST=SHA1 LOGINDISABLED XCONVERSATIONS COMPRESS=DEFLATE X-QUOTA=STORAGE X-QUOTA=MESSAGE X-QUOTA=X-ANNOTATION-STORAGE IDLE SASL-IR AUTH=PLAIN STARTTLS
google: IMAP4rev1 UNSELECT IDLE NAMESPACE QUOTA ID XLIST CHILDREN X-GM-EXT-1 UIDPLUS COMPRESS=DEFLATE
dovecot 2.0.19: IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS MULTIAPPEND UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS
Isode: IMAP4rev1 UIDPLUS LITERAL+ UNSELECT NAMESPACE SORT BINARY MULTIAPPEND THREAD=ORDEREDSUBJECT THREAD=REFERENCES ESEARCH LIST-EXTENDED IDLE URLAUTH CATENATE CONDSTORE QRESYNC ENABLE ACL RIGHTS=kxet QUOTA SASL-IR WITHIN CONTEXT=SEARCH SEARCHRES
Exchange 2007: IMAP4 IMAP4rev1 AUTH=NTLM AUTH=GSSAPI AUTH=PLAIN STARTTLS IDLE NAMESPACE LITERAL+

Exchange 2007: no support for custom keywords and no reports of keyword changes from other clients
