1) you have a database of homeowners (possibly public data) and (somehow) their e-mail addresses
2) for a fee you give this address to a user
3) the latter can thus cold e-mail the owner asking if he/she wishes to sell the house
So, if the thingy catches, homeowners of nice houses in good areas (and that have no intention whatever to sell their home) will be spammed by strangers.
BTW - and IMHO - the "conversion rate" of 90% opened letters are seemingly meaningless metrics (unless we have some comparison data on how many people opens "random" letters they receive), it would be interesting to know the "conversion rate" of letters actually leading to a sale.
Maybe it is a US thing, I wouldn't even think of sending letters to perfect strangers telling them I would like to buy their house (and possibly making an offer for a house I have not even visited).