mailbox has a name, optionally a route (not used nowadays), and
a formal address specification.
new mailbox ~name route addr_spec
route =  if not used (formerly, routes were used to specify
the way the mail should take from the sender to the receiver, and
contained a list of hostnames/IP addresses).
method name :
The name of the mailbox. Raises
Not_found if not set
method route :
The route to the mailbox
method spec :
The formal address specification