I spent an hour on the phone with the most understanding and compassionate Snow. He picked my brain about Plexus v2.  When he was finished, he sent this wonderful chart along, which I’ve posted here to help clarify things.

sending
front end / api creates rfc2822 message
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plexus_smtp - the message router
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|___plxaddr - decodes headers in the rfc2822 message
                     determining the type of message and the destination
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|	nq - queues the message for delivery ( stored in db )
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|___plex - looks up the destination and decides which sharer to use
                   and the appropriate handle
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| creates rfc2822 message
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[service]_sharer - receives rfc2822 message .
                     handles the delivery of the message to the specified service


receiving
[service]_listener - retrieves messages from the specified service .
                       these might be broadcast or direct messages .
                       creates an rfc2822 message with a json body and delivers to...
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plexus_smtp - the message router
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| \
|___plxaddr - decodes headers in the rfc2822 message
              determining the type of message and the destination
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|___plex - looks up the message sender and origin
              and decodes it into a contact
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| \
|	dq - queues the message for receipt ( stored in db )
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| creates rfc2822 message
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front end / api - receives rfc2822 message