class mailbox :?name:string -> string list -> addr_spec ->object..end
mailbox has a name, optionally a route (not used nowadays), and
 a formal address specification.
 Create a mailbox with
  new mailbox ~name route addr_spec 
 Pass 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 : stringNot_found if not setmethod route : string listmethod spec : addr_spec