Mortgage deliveries into Fannie Mae and Freddie Mac securities pools slowed measurably in the fourth quarter, with Fannie taking the biggest downturn, according to a new ranking and analysis by Inside Mortgage Finance. Fannie Mae issued $122.49 billion of new single-family MBS during the fourth quarter, a sharp 21 percent drop from the previous period, the analysis shows. The government-sponsored enterprise continued to rely heavily on its top lender customers for new product: a…