As the US housing slump deepens, troubled mortgage lenders are getting used to a new, unsettling sound. Jingle-mail - the clink of housekeys being mailed in by homeowners who are opting out of mortgages - is fast becoming a buzzword in the industry.

Borrowers who overstretched themselves to finance the purchase of a home have been hit hard by plunging house prices and a tightening of credit standards. For many, foreclosure appears to be the only way out.