Here's my analysis from four months ago (an excerpt of a thread "unrolled" on Reddit in January). It appears to have just been confirmed by the New York Times (see last tweet). Link to full thread: reddit.com/r/RussiaLago/c… 
2/ The NYT is being quite silly in saying Mashburn's testimony can't be corroborated. Of course it can be: by George Papadopoulos, the man who sent Mashburn the email. There's NO WAY Mueller would've followed up this lead if Papadopoulos hadn't already told him the email existed.
3/ Indeed, if Papadopoulos—who's pleaded—were to have told Mueller that he never emailed Mashburn any such email, Mueller would've been more likely to begin an investigation of Mashburn for lying to Congress than to begin searching for the email Mashburn spoke of. But he didn't.
4/ So I don't *care* whether Mueller has found Mashburn's email yet; the mere fact that he's *searching* for it means that Papadopoulos already told him it exists and would testify accordingly in any impeachment trial—which is a *huge* Trump-Russia smoking gun the NYT is missing.
