In the IMAP protocol the user can assign flags to mails (or other objects). For example, a flag is assigned to a message to indicate that it has been read (\Seen), it can be marked as important mail or it can be indicated that it has already been answered (\Answered). The usable flags are not predefined in the protocol, IMAP clients can assign any flags they desire.

Flags can be controlled similarly to requests and responses using the flag hash. It is a normative hash indexed by the name of the flag (case sensitive). The common practice is to accept any flags by default and explicitly drop unneeded flags. The possible actions related to flags are shown in the table below.