Sara siad that because the guy on the Mancow show that gave the tax return to Mancow opined the same thing.
https://www.nytimes.com/2017/03/15/us/politics/rachel-maddow-trump-tax-return.html?_r=0
....The journalist who obtained the records, David Cay Johnston, a former tax reporter for The New York Times, said that the documents arrived “over the transom” in his mailbox. Mr. Johnston even speculated on-air that Mr. Trump had sent the documents himself.