The Washington Post has apparently obtained some e-mails from high up in the Trump campaign, including one of their foreign policy advisors trying to set up a meeting with Putin. It was met with concerns by at least one Trump advisor (Real Adm. Charles Kubic) saying that it could be a violation of the Logan Act. Paul Manafort was on these messages, and expressed similar concerns.

He (via his lawyer) is now trying to use this to show that this is proof that there was no illegal activity by the campaign with respect to meetings with Russians and the like.

The problem, though, is that this all took place before Manafort met with a lawyer with close ties to the Russian government. So if he's ever charged, this is proof that he knew that meetings with Russian-connected folks could be a problem. In the words of one former prosecutor quoted by Business Insider:

If anyone on those emails later met with Russians or accepted aid from them, the prior emails about concerns could be used to indicate that they knew that the meeting was problematic and potentially illegal but nonetheless persisted.

Interestingly, Business Insider suggested that the fact that Mueller was able to get a search warrant for Manafort's home "indicates that Mueller managed to convince a federal judge that Manafort would try to conceal or destroy documents subpoenaed by a grand jury."