If there were any lingering doubts that the residential mortgage-backed securities market is making a strong comeback this year Commonwealth Bank put them to rest when it completed its latest Medallion Trust transaction on Friday. The bank issued a whopping A$4 billion and priced the senior notes at 70 basis points over the bank bill swap rate - the tightest credit spread in the RMBS market since the financial crisis.The bank last issued in February, when it raised $2.5 billion and paid 80 bps over the swap rate on the senior notes.Westpac paid 78 bps on the senior notes of an RMBS issue in May, while ME Bank paid 75 bps on the senior notes of an issue last month.At the time of the ME Bank deal, the bank's general manager markets, John Caelli, said the fall in funding costs this year reflected strong demand in the financial market for assets generating yield."If you look at the credit market, the RMBS segment has lagged for some time. What we are seeing now is a catch-up," he said.CBA's latest $4 billion issue passes the bank's previous record $3.2 billion for RMBS issuance, raised last year.The $3.68 billion of class A notes, which have a weighted average life of 2.8 years, were priced at 70 bps over the one-month bank bill swap rate.Pricing was not disclosed on the $240 million of B notes, which have a weighted average life of 5.2 years, and the $80 million of C notes, which have a weighted average life of 9.4 years.