IMAP "$Important" Keyword and "\Important" Special-Use Attribute
Extracted elements (12)
"$Important" is deliberately distinct from "\Flagged" in two ways: it conveys general importance for triage rather than urgency or follow-up, and its setting is expected to be driven at least partly by server-side heuristics rather than exclusively by explicit user action.
The registration policy for the IMAP Mailbox Name Attributes registry is intentionally permissive: Expert Review is meant to be close to first-come-first-served, with the expert only checking that the reference and description are adequate for interoperable implementation, not blocking reasonable requests.
The "IMAP Mailbox Name Attributes" registry is shared with the JSON Meta Application Protocol (JMAP) for Mail. Several attributes from RFCs 3501 and 5258 are annotated "not used by JMAP" in the Usage Notes field.
The "$Important" keyword is registered as a PRIVATE keyword in the IMAP Keywords registry; it is advisory but may influence automatic server actions as described in the specification.
The "$Important" keyword is a signal that a message is likely important to the user. It is generally expected to be set automatically by the system based on heuristics (sender, recipients, subject, content), though users may also set it manually.
The "\Important" mailbox attribute designates a special-use mailbox containing messages likely important to the user. In implementations supporting "$Important", it typically represents a virtual mailbox collecting messages marked with that keyword from other mailboxes.
RFC 8457 creates the "IMAP Mailbox Name Attributes" registry under the IMAP registry group. Registration policy is IETF Review for IETF consensus documents or Expert Review for other submissions; attributes are registered without the leading backslash and treated as case-insensitive US-ASCII.
RFC 8457 registers the "$Important" keyword in the existing "IMAP Keywords" registry (established by RFC 5788), classifying it as PRIVATE, advisory, with COMMON intended usage and IESG as the change controller.
The IMAP Mailbox Name Attributes registry is initially populated with attributes from RFCs 3501, 5258, 6154, and this document: All, Archive, Drafts, Flagged, HasChildren, HasNoChildren, Important, Junk, Marked, NoInferiors, NonExistent, Noselect, Remote, Sent, Subscribed, Trash, Unmarked. Attributes marked with "*" carry Usage Note "not used by JMAP".
Identifying important messages or the mailbox holding them gives an attacker a strategic starting point. If the importance algorithm is well known, additional information may be inferred — for example, that senders of "$Important" messages are particularly significant to the mailbox owner.
Implementations should protect the IMAP channel from passive eavesdropping to prevent unauthorized discernment of a user's "\Important" mailbox or "$Important" messages; use of IMAP STARTTLS (RFC 3501 Section 11) is recommended.
The "\Important" mailbox attribute extends the ABNF from RFC 6154 Section 6 with the production: use-attr =/ "\Important". It is used in LIST responses and CREATE commands with the USE parameter.