Do you have any issues with other email - ie, do you get all of those
ShopDisney promos?
Hopefully, you're right about the mail, but it's possible DVC uses a separate postal service. (I think
D23 eventually got their own.) It used to be way more obvious if there were mail issues - I could tell which month my problem started because I immediately stopped getting the Mickey Monitor mailing that used to come with an AP (and started getting it again after the "fix"). I don't think I've received anything other than passholder magnets from Disney in the last couple of years.
I actually thought about suggesting that - certainly couldn't hurt, and might reset whatever the sticking point is.