public string SignEmail(string message)
{
// get email content and generate initial signature
var email = Email.Parse(message);
email.Headers.Add(SignatureKey, new EmailHeader { Key = SignatureKey, Value = this.GenerateDkimHeaderValue(email) });
email.Headers[SignatureKey].Value += this.GenerateSignature(email);
return SignatureKey + ':' + email.Headers[SignatureKey].Value + Email.NewLine + email.Raw;
}