Facebook will soon rely on centuries-old technology to try to prevent foreign meddling in U.S. elections: the post office.

Baffled in 2016 by Russian agents who bought ads to sway the U.S. presidential campaign, Facebook’s global politics and government outreach director, Katie Harbath, told the National Association of Secretaries of State on Saturday that the company would send postcards to potential buyers of political ads to confirm they reside in the U.S.

The recipient would then have to enter a code in Facebook to continue buying the ad. The method will first apply to ads that name candidates ahead of the midterm elections in November, says Facebook spokesman Andy Stone.

The plan was unveiled a day after special counsel Robert Mueller charged 13 Russians with interfering in the presidential election.

(AP)