If I were you, I would stop responding to those emails and throw them away like your aunt and uncle do with the letters. Write him an email about how you love him as your father, and want a relationship, etc. but that you would like him to stop sending that material, and that you will not respond to them. Only respond to his emails that contain other material.
If he doesn't email you without talking about the WT, then you take the lead and periodically write to see how he is, shoot the breeze, etc. No mention of the WT. I would make it a moot point by not responding to the material. At this point in time, it sounds like he will not change. IF you want a relationship with him, you will need to set the boundary, and stop responding rewarding his behavior with your response. When you engage in debate, he still thinks there is a chance to win you over. IF you stop responding, there is nothing for him to struggle against, and the fight will eventually die off.
Just my 2 cents.
DS