Fannie Mae will no longer accept back a mortgage that was repurchased by a secondary market investor, government-sponsored enterprise or private institutional investor — even if the lender cured the defect in the loan. Major banks must repurchase a mortgage from Fannie Mae if it was not written to the GSE’s guidelines and went into default. These mortgage repurchase obligations could cost banks as much as $43 billion, according to recent estimates from Standard & Poor’s. According to servicer guidance issued Monday by Fannie Mae, the GSE allows the redelivery of a mortgage loan that was repurchased by the lender so long as the bank corrected the loan to fit Fannie’s underwriting standards. But the GSE clarified in the guidance that any mortgage that was repurchased by investors or another GSE such as Freddie Mac are not eligible to be sold back to Fannie Mae. However, a mortgage that a lender had to repurchase from another investor or GSE that was delivered in error to that investor or GSE can be sent to Fannie Mae if it meets all of its requirements. Even though S&P said the banks are roughly one-fourth through the combined buybacks, the credit rating agency does not see a threat to capital levels. “In all cases, we believe that the representations and warranties matter is an earnings issue and is not likely to affect our view of the banks’ capital adequacy,” S&P said. Write to Jon Prior.