As it delivers messages to local users PMDF checks to see if the
    user has a MAIL.DELIVERY file in their default login directory.
    DELIVER is invoked if this file exists. DELIVER takes the
    following steps:

    1. DELIVER reads and parses the MAIL.DELIVERY file.

       By default the message is returned to the sender if any errors
       occur during the reading and parsing of the MAIL.DELIVERY
       file. Note that an empty MAIL.DELIVERY file is considered an
       error.

       The system administrator can configure DELIVER to change this
       behavior. If the logical name PMDF_IGNORE_MAIL_DELIVERY_ERRORS
       is defined /SYSTEM/EXECUTIVE, any errors in the MAIL.DELIVERY
       file (including an empty file) are ignored. The mail is
       delivered normally to the user's NEWMAIL folder as if the
       MAIL.DELIVERY file did not exist.

    2. DELIVER writes the headers of the message to a temporary file
       in the recipient's home directory.

    3. DELIVER writes the body of the message to a temporary file in
       the recipient's home directory.

    4. A command file is constructed to complete the delivery
       process. This file is also created in the recipient's
       home directory. The directives previously read from the
       MAIL.DELIVERY file are compared with the message. Any
       directives that match will cause commands to be written to
       the command file that implements the requested action.

    5. After the list of directives is exhausted DELIVER checks to
       see that at least one directive caused an action to be taken.
       If none did, DELIVER writes to the command file a default
       action command to deliver the message normally. Commands to
       delete the message file (unless the MESSAGE_DELETE flag is set
       to NO by one of the actions) and the command file itself are
       written to the command file and the command file is closed.

    6. The command file is queued to the batch queue specified by the
       MAIL.DELIVERY file for processing. If the MAIL.DELIVERY files
       not specify a queue, the DELIVER_BATCH queue will be tried,
       and if that fails the queue SYS$BATCH will be used. The file
       is queued so that it will execute just as if the recipient had
       submitted it for processing from his or her own account. Once
       the command file is submitted DELIVER tidies up, deallocating
       any storage allocated for directive lists, and returns control
       to PMDF.

       DELIVER does not bother to create the batch job if there's no
       work for it to do.

    7. DELIVER passes responsibility for delivery back to PMDF if it
       was asked to deliver the message to the user's NEWMAIL folder
       and the requested handling of headers matches the the handling
       specified by the local channel. This does not preclude other
       actions using the message in other ways.