A KeyedMailbox object implements a message
mailbox indexed by a unique key string for each message. When a message is
saved into the store, the message's key is returned. Later, the key is used
to retrieve file name the message is stored in.
The message store has the following characteristics:
It is a Maildir, so various mail programs can read the messages directly
and without adversely affecting the mailbox.
The key is very hard to guess.
The key is short and can be included in a message subject or in the
extension of a return address (suitable for mailing list style
Saves a message into a confirmation queue and returns a string key that can
be used to retrieve it later. They key is a string of hex digits and
dashes, suitable for inclusion in a message subject or "VERP"