A potentially dangerous Request.Form value was detected from the client for mandrill webhook

Question

I am using this for handle bounce mails on our server

    [ValidateInput(false)]
    public string Mandrill(FormCollection fc)
    {
    string json = fc["mandrill_events"];
    }

but I am getting this error System.Web.HttpRequestValidationException (0x80004005): A potentially dangerous Request.Form value was detected from the client (mandrill_events="...550 5.1.1 ...").

Please help for solve this issue.