2.2 Message Syntax

A Message object can have its flag state changed by the client. Constraints for setting flags on a Message object are defined in section 2.2.1 through section 2.2.3.

When a value is specified as not present, the property MUST NOT exist on the Message object, and if it exists, it MUST be deleted. Setting a property to 0 or to a zero-length string does not delete the property from the Message object.