The deferred channel keyword implements recognition and honoring of the Deferred-delivery: header. Messages with a deferred delivery date in the future are held in the channel queue until they either expire and are returned or the deferred delivery date is reached. See RFC 1327 for details on the format and operation of the Deferred-delivery: header.
nodeferred is the default. It is important to realize that while support for deferred message processing is mandated by RFC 1327, actual implementing of it effectively lets people use the mail system as an extension of their disk quota.