§ 1414 [Authentication by Party Admission or Conduct]

If you want to prove a document is real and legitimate, one easy way is to show that the other side already treated it that way. This rule covers two situations where that happens.

First, if the person you're up against has ever admitted the document is genuine, that admission can be used to prove it's authentic. It doesn't matter when they said it, as long as they said it at some point.

Second, if that person acted like the document was real, even without saying so directly, that can also count as proof. For example, if someone received a letter and then responded to it or followed instructions written in it, their actions show they believed the letter was genuine. Courts allow this kind of evidence because people usually don't act on documents they think are fake.