IMAP4rev2

Plugin	IMAP4rev2
Capability	IMAP4rev2, advertised next to IMAP4rev1
RFC	RFC 9051
Loads	ENABLE, NAMESPACE, UNSELECT, UIDPLUS, ESEARCH, SEARCHRES, IDLE, SASL-IR, LIST-EXTENDED, LIST-STATUS, MOVE, BINARY, SPECIAL-USE, STATUS=SIZE, AUTH=PLAIN, and LITERAL- unless LITERAL+ is loaded

ImapKit implements IMAP4rev2 the way RFC 9051 Appendix A describes for a server that supports both revisions: it advertises IMAP4rev1 and IMAP4rev2, every session starts as IMAP4rev1, and a client that wants IMAP4rev2 sends ENABLE IMAP4rev2. One server can therefore test an IMAP4rev1 client and an IMAP4rev2 client side by side, and a client that supports both can be checked for picking the right one.

const server = imapkit({ plugins: ['IMAP4rev2'] });

Capabilities

The plugin loads every extension that RFC 9051 Appendix E item 2 folds into IMAP4rev2, and each of them advertises its own capability. AUTH=PLAIN is required by RFC 9051 section 6.1.1.

C: A0 CAPABILITY
S: * CAPABILITY IMAP4rev1 ENABLE NAMESPACE UNSELECT UIDPLUS ESEARCH SEARCHRES IDLE SASL-IR LIST-EXTENDED LIST-STATUS MOVE BINARY SPECIAL-USE STATUS=SIZE AUTH=PLAIN IMAP4rev2 LITERAL-
S: A0 OK Completed

• LITERAL- (non-synchronizing literals up to 4096 octets, RFC 9051 section 4.3) is loaded only if neither LITERAL+ nor LITERAL- is loaded already. Load LITERAL+ as well to allow non-synchronizing literals of any size, it replaces the implied LITERAL- in any load order. Listing LITERAL- and LITERAL+ by name still throws.
• STARTTLS and LOGINDISABLED (RFC 9051 section 6.1.1) are not loaded, as they change how clients log in. Load them yourself when you need them.
• ENABLE IMAP4rev2 does not change the CAPABILITY list.

Sessions before ENABLE

A session that does not send ENABLE IMAP4rev2 behaves like it would on an IMAP4rev1 server with the same extensions. It gets RECENT, [UNSEEN n] and plain SEARCH responses, can use CHECK, LSUB and the RFC822 FETCH items, and 8-bit characters in quoted strings are refused (RFC 9051 Appendix A). Partial FETCH ranges and the LARGER and SMALLER search keys take 32-bit numbers (RFC 3501 section 9).

C: A3 SELECT INBOX
S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft \Deleted \Seen \*)] Flags permitted
S: * 4 EXISTS
S: * 0 RECENT
S: * OK [UNSEEN 2] First unseen message
S: * OK [UIDVALIDITY 1] UIDs valid
S: * OK [UIDNEXT 5] Predicted next UID
S: A3 OK [READ-WRITE] Completed
C: A4 SEARCH UNSEEN
S: * SEARCH 2 3 4
S: A4 OK SEARCH completed

Sessions after ENABLE IMAP4rev2

ENABLE IMAP4rev2 must come before the first SELECT or EXAMINE (RFC 5161 section 3.1), as for every ENABLE. After it, the session follows RFC 9051:

Area	IMAP4rev2 behavior
SELECT, EXAMINE	No RECENT response (nor after the EXISTS of new messages) and no [UNSEEN n] code. An untagged LIST response for the selected mailbox, with its special-use attributes. * OK [CLOSED] when a mailbox was selected before (section 6.3.2)
\Recent	Not sent in FLAGS of FETCH responses (section 2.3.2)
SEARCH	Answers with an ESEARCH response, RETURN (ALL) when no result option is given (section 6.4.4). UTF-8 is assumed without CHARSET, and CHARSET is still allowed
STATUS	DELETED is allowed (section 6.3.11), RECENT is not
Strings, mailbox names	UTF-8 in quoted strings and mailbox names, which must be Net-Unicode in Normalization Form C (section 5.1)
APPEND	A message with 8-bit header fields can be appended (section 6.3.12)
message/global	Described in BODYSTRUCTURE and numbered in sections like message/rfc822
Numbers	Partial FETCH ranges and LARGER/SMALLER take 63-bit numbers (number64)

Items that RFC 9051 removed (Appendix E) are refused with BAD: CHECK (use NOOP), LSUB (use LIST (SUBSCRIBED)), the RFC822, RFC822.HEADER and RFC822.TEXT FETCH items (use BODY[], BODY.PEEK[HEADER], BODY[TEXT]), the NEW, OLD and RECENT SEARCH keys and the RECENT STATUS item.

C: A2 ENABLE IMAP4rev2
S: * ENABLED IMAP4rev2
S: A2 OK ENABLE completed
C: A3 SELECT INBOX
S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft \Deleted \Seen \*)] Flags permitted
S: * 4 EXISTS
S: * OK [UIDVALIDITY 1] UIDs valid
S: * OK [UIDNEXT 5] Predicted next UID
S: * LIST (\HasNoChildren) "/" INBOX
S: A3 OK [READ-WRITE] Completed
C: A4 SEARCH UNSEEN
S: * ESEARCH (TAG "A4") ALL 2:4
S: A4 OK SEARCH completed
C: A6 CHECK
S: A6 BAD CHECK is not part of IMAP4rev2, use NOOP (RFC 9051 Appendix E)
C: A7 FETCH 1 (RFC822.HEADER)
S: A7 BAD RFC822, RFC822.HEADER and RFC822.TEXT are not part of IMAP4rev2, use BODY[] (RFC 9051 Appendix E)
C: A8 SEARCH RECENT
S: A8 BAD Invalid search key RECENT
C: A9 STATUS Archive (MESSAGES DELETED SIZE)
S: * STATUS Archive (MESSAGES 0 DELETED 0 SIZE 0)
S: A9 OK Status completed
C: A11 LSUB "" "*"
S: A11 BAD LSUB is not part of IMAP4rev2, use LIST (SUBSCRIBED) (RFC 9051 Appendix E)
C: A12 SELECT Sent
S: * OK [CLOSED] Previous mailbox closed
S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft \Deleted \Seen \*)] Flags permitted
S: * 0 EXISTS
S: * OK [UIDVALIDITY 1] UIDs valid
S: * OK [UIDNEXT 1] Predicted next UID
S: * LIST (\HasNoChildren \Sent) "/" Sent
S: A12 OK [READ-WRITE] Completed

ENABLE matches capability names case-insensitively and lists them in the advertised spelling, so ENABLE imap4rev2 is answered with * ENABLED IMAP4rev2. With the UNAUTHENTICATE plugin, UNAUTHENTICATE turns IMAP4rev2 off again, as it does for every enabled extension (RFC 8437 section 4.1).

Keywords

RFC 9051 section 2.3.2 asks servers to keep the $Forwarded, $MDNSent, $Junk, $NotJunk and $Phishing keywords. With the plugin loaded, a mailbox that does not allow new keywords (no \* in PERMANENTFLAGS) still lists these five in PERMANENTFLAGS and keeps them on its messages.

Not implemented

• A server that advertises only IMAP4rev2.
• UTF-8 in response text.
• The OLDNAME extended data item of LIST responses (section 6.3.9.7). Only NOTIFY sends it, for RENAME.

Other extensions that need IMAP4rev2 semantics, like UTF8=ACCEPT, share the same session state, so the two plugins work together in any load order.