N.H. Rev. Stat. § 80:28
The purchaser of any real estate sold by a collector of taxes, within 45 days from the date of such sale, shall notify all persons holding mortgages upon such property as recorded in the office of the register of deeds. In the event that a person holds a mortgage on more than one piece of property, a listing of the property may be forwarded by the purchaser. Whenever a town becomes such a purchaser and the selectmen thereof determine that one or more outstanding mortgages exist, they may direct the collector of taxes to give such notice to any mortgagee, and the collector shall thereupon be entitled to receive the same fees as provided in RSA 80:37 for notifying any mortgagee of a payment after sale. Such notice shall give the date of the tax sale, the name of the delinquent taxpayer, the total amount for which said real estate was sold and the amount of costs for notifying mortgagees. As provided in RSA 80:37, the tax collector shall send a similar notice to any mortgagee within 30 days of the time of payment of any subsequent tax thereon by the purchaser. Any tax sale of such encumbered real estate shall be void as against any mortgagee and no tax collector's deed based on said sale shall be valid unless the mortgagees shall have been notified in the manner provided in RSA 80:29, but the tax and any subsequent tax payments made upon the property shall be collectible and payment may be enforced by suit under the provisions of RSA 80:50.
RSA 80:28
1887, 92:1. PS 61:8. 1907, 120:1. 1925, 61:1. PL 66 :25. RL 80:25. 1947, 187:1. 1949, 155:1. RSA 80:28. 1983, 285:2. 1985, 312:6, eff. Aug. 13, 1985.