Often, multiple IMAP (RFC 3501) clients need to coordinate changes to a common IMAP mailbox. Examples include different clients working on behalf of the same user, and multiple users accessing shared mailboxes. These clients need a mechanism to efficiently synchronize state changes for messages within the mailbox. Initially defined in RFC 4551, The Conditional Store facility provides a protected update mechanism for message state information and a mechanism for requesting only changes to message state. This memo updates that mechanism and obsoletes RFC 4551, based on operational experience. This document additionally updates another IMAP extension, Quick Resynchronization, which builds on the Conditional Store extension to provide an IMAP client the ability to fully resynchronize a mailbox as part of the SELECT/EXAMINE command, without the need for additional server-side state or client round-trips. Hence this memo obsoletes RFC 5162. Finally, this document also updates the line length recommendation in Section 3.2.1.5 of RFC 2683.