Mail Queue And Delivery System

FML::Process::QueueManager picks up one queue from the queue directory. Mail::Message parses it. Mail::Delivery processes the real delivery via FML::Mailer.

Mail::Delivery::Queue
   |
   | ---> queue directory
   V
FML::Process::QueueManager
   |
   | <--- queue directory
   V
FML::Mailer
   |
   V
Mail::Delivery

In manipulating queue, we should lock the target queue by flock(2).