IMAP Extension for Object Identifiers

This document defines the OBJECTID+ extension for IMAP, which obsoletes [RFC8474]. OBJECTID+ introduces a compound OBJECTID response format that bundles object identifiers into key-value pairs, an ACCOUNTID identifier for account-level context, OBJECTID response codes for the RENAME command, and identifier-based mailbox selection via SELECT and EXAMINE. The OBJECTID+ extension is activated implicitly when a client uses any OBJECTID+-specific feature, ensuring backward compatibility with clients that only support [RFC8474]. This document also updates [RFC9698]: when JMAPACCESS is advertised alongside OBJECTID+, ACCOUNTID values MUST correspond to JMAP accountIds.