See SYNOPSIS in Mail::Message
Dummy messages are used by modules which maintain ordered lists of messages, usually based on message-id. A good example is Mail::Box::Thread::Manager, which detects related messages by scanning the known message headers for references to other messages. As long as the referenced messages are not found inside the mailbox, their place is occupied by a dummy.
Be careful when using modules which may create dummies. Before trying to access the header or body use isDummy() to check if the message is a dummy message.
See DESCRIPTION in Mail::Message