Legal research help, juror post communication.

There was a trial with jury on Monday and the plaintiff's lawyer win the case, however, few days later the plaintiff's lawyer got a mail with a love letter from one of the Juror. My question is that is there any cases or law in the USA federal court to tell what the plaintiff's lawyer should do? Any circuit court case that show the process of what should be happen, would be helpful. For example, does the law need to report it to the court and notify the defendant lawyer? or will the court call the Juror into courthouse and ask if the juror was bias or not.