I have a startup idea that involves offering a service to people in the U.S. who recently took out mortgages. I feel as though mailing lists of such people must be available somewhere (from the lending banks, perhaps), but I've not been able to find them. Can anybody point me in the correct direction?
Let me ask you a different question: why do you want to build a mailing list yourself, especially since this mailing list will keep going out of date (presumably X months after the mortgage they are no longer your target)?
Far better would be to do targeted ads with things like Google or Facebook. People who are about to take out mortgages will almost certainly be visiting appropriate websites, which will set up lovely advertising cookies, allowing you to serve your ads to the right customers without knowing who they are.
The one challenge you would face is you may want to time delay the ads, to show up X many weeks after your customers have searched for mortgages: I don't know how you would organize that with Google or Facebook ... and to an extent that is their problem. They just want to give you good click-through rates, so they will get paid!!
P.S. I realise mortgages are probably expensive keywords, and if you look at the effort required in other suggested solutions, you may find that the expense is cheaper than alternatives.