Derp state insider was very likely planted inside Mannafort's circle of "friends" and had access to the account.
Or, obviously, one of those dudes flipped--likely his assistant, Rick Gates is it? that was the 2nd guy indicted, with Mannafort at #1, to get the ball rolling on all of this. I think that guy had very little money, relatively, and can't afford the type of defense that Mannafort can with his criminal treason-obtained wealth, so had no other out.
Or the sooper sekret email account and password was locked away in Cohen's office safe. 12345 is a tough password to crack.