Chance I actually had a similar thing happen to me many years ago. The entire problem stemmed from me mentioning delinquent taxes on the postcard. This got a few people excited (and not in a good way).
As a general rule, if you’re going to call out anything specific about delinquent taxes, make sure it’s in an enclosed envelope (or just don’t mention it at all... if someone’s got delinquent taxes, they already know who they are).
The wording of my postcards is intentionally vague about this, for this very reason... but if you really want to steer clear of any issues, you can just not mention it.