public SendGridEmailSender(IMailHostSettingsProvider hostSettingsProvider)
        {
            if (hostSettingsProvider == null)
            {
                throw new ArgumentNullException("hostSettingsProvider");
            }

            this.hostSettingsProvider = hostSettingsProvider;
        }
        public SendGridPostman(IMailHostSettingsProvider hostSettingsProvider, IRenderer htmlRenderer)
        {
            if (hostSettingsProvider == null)
            {
                throw new ArgumentNullException("hostSettingsProvider");
            }

            if (htmlRenderer == null)
            {
                throw new ArgumentNullException("htmlRenderer");
            }

            this.hostSettingsProvider = hostSettingsProvider;
            this.htmlRenderer = htmlRenderer;
        }