Facebook plans to mail out verification postcards through the US postal service to anyone purchasing ads related to US elections, according to Reuters. The postcards will contain a special code that advertisers must provide back to Facebook to prove they’re in the United States.

This new verification system will be required for all advertising that mentions a specific candidate running for a federal office — such as the presidency. It will be implemented in time for the mid-term elections this coming November.

“If you run an ad mentioning a candidate, we are going to mail you a postcard and you will have to use that code to prove you are in the United States,” Katie Harbath, Facebook’s global director of policy programs, said on Saturday. She acknowledged to Reuters that the process “won’t solve everything” but is just one step Facebook is taking to prevent foreign nationals from spending money under fake cover to influence US politics. (The verification method won’t be required for ads that focus on political issues — only for those that mention a specific candidate.)

The news comes a day after special counsel Robert Mueller pulled back the curtain on Russia’s troll farm operation during the 2016 election. His indictment reveals a startling, sophisticated, and sustained campaign designed to influence the 2016 presidential election through false personas that Russians used to spread divisive political content over social media, among other tactics.