Email that a spam filter cannot analyze sufficiently to determine if it is legitimate or not. Graymail is often not spam, but may be newsletters that people once subscribed to or product offerings to customers who purchased from the vendor before. See spam
The original term refers to the threat of releasing classified government documents in order to influence legal proceedings. To counter graymail, the 1980 U.S. Graymail Law allows judges to review classified material in secret.