State Department officials regularly mark some information as “sensitive but unclassified” (SBU), and there are special rules to deal with this.
Foreign Affairs Manual (FAM) rules state that anyone regularly transmitting SBU information outside the department’s OpenNet computer network needs to request a solution from the department’s security officials. Clinton never does this, even though she frequently sends and receives emails marked SBU.
Furthermore, rules require special safeguards for transmitting SBU information on a mobile device. Clinton never does that either. (US Department of State, 5/25/2016)