The keys of %msg are message numbers.  Since %msg contains records for
only those messages that are active in the queue at a particular time,
there is no collision between keys.  If we wanted to store historical
information about messages, we would need to use different keys.

The values of %msg are references to structures that represent the messages.
A typical structure looks like this:  *
                                      |
                                      |
          Message                     V
        +-----------------------------------------+
        | arrived  => arrival time in Unix format |
        | msg      => message number              |
        | bounced  => 'B' or '-'                  |
        | length   => length in bytes             |
        | from     => sender's address            |    +--------------+
        | del      =>  *------------------------------>|123456 => *--------+
        | finished => removal time in Unix format |    | 23987 => *------+ |
        +-----------------------------------------+    |   ...    ... |  | |
                                                       |(One entry for|  | |
                                                       | each delivery|  | |
                                                       | attempt.)    |  | |
    Delivery attempt                                   +--------------+  | |
  +------------------------------------------+                           | |
  | lr       => recipient is local or remote |                           | |
  | to       => recipient address            |<--------------------------+ |
  | started  => delivery attempt time        |                             |
  | finished => completion time              |   +--------------+          |
  | result   => success/deferral/failure     |   |lr => ...     |<---------+
  | report   => additional messages          |   |to => ...     |
  +------------------------------------------+   |...   ...     |
                                                 +--------------+