Clinton’s deputy chief of staff Huma Abedin sends an email to Justin Cooper, an aide to Bill Clinton. She forwards a message from Clinton’s chief of staff Cheryl Mills to Clinton that had bounced, and asks Cooper, “HRC [Clinton] email coming back—is server okay?”
He replies, “UR [You are] funny. We are on the same server.” His reply goes to Mills as well as Abedin, indicating that both of them are aware of the existence of Clinton’s private server.
Cooper’s email domain will be redacted when this email is released in 2016, but his comment indicates his email is on the clintonemail.com domain, the same as Clinton’s. (Abedin has an email account on that domain too, but she sends this email from her State Department state.gov account.) (US Department of State, 6/20/2016)
Cooper is not a government employee and apparently has no security clearance, but other reports indicate he helps Bryan Pagliano manage Clinton’s server.