Nextdoor is a legitimate company. There is a page about them here on Wikipedia https://en.wikipedia.org/wiki/Nextdoor Its registration process is onerous: They require: Postcode, first line of address, email address, mobile phone number For first line of street address, they wish to confirm it via either a phone call to your mobile or a postcard sent to your address. There is a note saying "We'll automatically mail you a verification postcard if we can't confirm your address another way." I wonder if someone might have created an account and given your address and you have just received said postcard after a while.