Basically even if user expressly add you to their contact list and pull you out of spam, etc, if you sending them just your email ignoring what they reply back to you, eventually you get back to spam. I think this is very clever and is based on simple human rule that most follow when conversating over email: when people reply to email the do not start a new email, they hit reply and previous email from their convo lands on the bottom of this new email.
Once we started emailing new emails with adding content from addessee previous email to the bottom, our emails start getting back to addressee major mailbox.
And this make perfect sense on how to detect a real conversation (gmail has your sent items so if incoming email contains something you wrote before, its an indication you have a real live conversation with someone)