The DELETE command is used to delete one or more messages from
    the channel queue directories. The messages to be deleted are
    specified by their message identification numbers shown by
    the most recent DIRECTORY command. That number appears in the
    leftmost column of the DIRECTORY command listing. Ambiguous
    message numbers must be qualified by the proper channel name
    with the /CHANNEL qualifier.

    Note that the DELETE command irrevocably deletes each message
    it is instructed to delete: the messages are not returned to
    their originators nor will any further attempts to be made to
    deliver them to their recipients. The messages are permanently
    deleted. Often, it is preferable to use the RETURN command so as
    to return the message to its originator, (e.g., bounce it back to
    the sender).