I was also quite impressed by the tone of the message from Mailchimp legal - probably the clearest, most polite takedown request I've seen.
If this situation escalated it could look sloppy on their part if they didn't handle it properly with the correct resources from the beginning.
There is no reason why this issue at hand can't be handled in a professional manner - in this case by an actual lawyer.
I've received a number of C&D and this to me is by far the most level-headed and polite that I've seen in a long time.
Just because you didn't expect a response form a lawyer doesn't mean its "aggressive" by default.
Good reading:
http://en.wikipedia.org/wiki/Miss#Use_as_a_title_.28honorifi...
http://www.cs.virginia.edu/~evans/cs655/readings/purity.html
PS: In the U.S. (if not the rest of the English speaking world?), if you want to use an honorific for a woman but don't know which she uses, always use "Ms.", never ever "Miss." "Miss" implies a woman is unmarried; "Mrs." implies that she is married and the surname is her husband's; "Ms.", appropriately for professional communication, makes no assumptions about her marital status.