Dictionary<string[], string> d = new Dictionary<string[], string>();
d.Add(new[] { "email@yahoo.com", "test@yahoo.com" }, "Group 1");
d.Add(new[] { "myemail@gmail.com", "checkit@gmail.com" }, "Group 2");

string keycheck = "email@yahoo.com";
string val = "";

d.TryGetValue(keycheck, out string k);