The deferred channel keyword implements recognition and honoring of the Deferred-delivery: header line. 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 line.
The keyword nodeferred is the default. It is important to realize that while support for deferred message processing is mandated by RFC 1327, actual implementation of it effectively lets people use the mail system as an extension of their disk quota and opens the system up to potential denial-of-service attacks on your disk space by arbitrary malicious users.