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).
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