Sandy's answer is a good reminder - be sure to keep the Post Office's Proof of Delivery in your file in case you ever need to prove you sent the Notice. Under the statutes, the notice is to be sent to "the addresses set forth in the municipal tax records" so that's the address to use. Although if you suspect they may have moved, and you can find out what their new address is, it's worth sending it to both, to make sure you both fulfill the law and make sure they actually receive it. We had one person who had not been paying, and we finally found out she had a new address. We mailed notice there, and wonder of wonders, she is now paid in full!