I got the yellow card in the mail too. I know there are people out there that get scared when they see signs like this one. My advise is always don't just look at the mail and react!. Read it twice and do a search on it if you are not sure. Also the first place to look for a company info is the Better Business Bureau. I did that with this company and they install water systems for house that don't have city water. This company had 45 complains in the last 3 years. They try to get you with a free water test at home. The way they advertise is deceiving, especially older people. Also, check for phone numbers that are registered as scam or telemarketers.
I got a letter in the mail that says "message for new home owner, we have not been able to verify the residents of this house, whom have a $50 amazon gift card able to be claimed. Please call this number to clams your amazon gift card." Even though I have lived in ny house for 10 yrs now, so it does not make sence. Obviously it is spam.
Most likely one of the water filtration scams. They come to your house to "personally" deliver this card, and then want to "analyze" your water and sell you a filtration system you don't need and that will certainly be way overpriced.