Here you go, proof of knowledge, lying about it to cover it up, and huma pointing out a docu This alone puts him in federal prison for 30.
When caught, he says he didn't do it; knowing about it is negligance to try and avoid 18 U.S. Code § 793.
https://hotair.com/archives/2016/09/24/obama-used-pseudonym-e-mail-hillary-secret-server/
He is found to have lied about it that.
https://www.politico.com/story/2016/09/hillary-clinton-emails-fbi-228607
This one essentially confirms the presence of sensitive materials on the server.
https://www.rt.com/usa/360528-obama-implicated-clinton-email/
Obama's involvement in knowing about the server and lying implies conspiracy, which just makes it all worse for him.