I don't know if I should care about being a star seller at all. But just for the record, I answer my emails immediately; however, my rating is 50% on email responses. I don't even know why that is. People send me junk mail, and I need to remember to mark it as junk. This is unfair. I Wish this algorithm was a little smarter. Should I care?
Emails have nothing to do with star seller.
@NineAmulets: As @NanaLetha said, only Etsy messages count for Star Seller. The Star Seller metric is that the FIRST message of EVERY NEW INCOMING thread must be replied to within 24 hours. It does not matter who sent it, the subject matter, if it is a duplicate or blank or anything else. If it is SPAM then it should be marked as such.
Etsy is now putting a blue text warning / advisory (blue banner on desktop, and a blue dot on mobile) on those messages they consider to be a "new message" but should semantically be called a "new thread".
If the unanswered message is not a Help Request, at least for the time being, it appears that marking an unanswered message as SPAM (even after the 24 hour deadline has passed) will cause your reply percentage to change in a day or two (but not retroactively) if the original message was within the current star seller period. Note that subsequent messages from that account may go directly to the SPAM folder.
The algorithm can't figure out what is spam without you telling it. Marking something as spam takes 5 seconds.
Remember, it is against Etsy rules not to reply to messages in a timely fashion, so should your shop get in other trouble, this will be held against you. Don't give Etsy a reason to put you on reserve or suspend you.