The review, which is in its early stages, is being handled through the intermediary Agency Insight.
E.ON began informal discussions with direct marketing agencies at the beginning of the year, but the process did not progress to a pitch.
However, E.ON is now expected to ramp up its direct marketing activity after its recent tie-up with Tesco.
E.ON and Tesco are taking on British Gas and Sainsbury's by offering their customers the opportunity to pay their utility bills using Tesco Clubcard vouchers.
E.ON currently uses a number of regional agencies on its direct marketing activity. Before this, Tequila\London had worked on the direct business as part of an integrated arrangement with its sister agency TBWA\London. However, TBWA lost the account to DLKW Lowe in 2009.
The energy company uses LBi for its digital activity. The agency won the account last year to work across digital creative, user experience, brand communications and mobile activity.
LBi is thought to be unaffected by the direct marketing review.
Last year, E.ON launched its first TV ad in two years using the strapline: "Energy fit." The spot was created by DLKW Lowe and encouraged consumers to improve their energy efficiency and reduce bills. The campaign included print, radio, direct marketing and digital activity.