OP is on flaky legal ground reproducing and distributing other's designs without permission as it is; but then to suggest that MailChimp is in the wrong, simply for, in a very polite, even friendly, letter, acknowledging their brand and their legitimate copyright, and asking OP to play fair...wow.

Also, he wanted someone from MailChimp's design or development team to reach out to him? Really? Maybe they're busy, you know, designing and developing things! This is not a design or development issue. They have legal council to worry about copyright and brand dilution issues.