private async void PollClient()
        {
            while (true)
            {
                var state = _client.CheckMailbox();

                if (state == MailState.InvalidCredentials)
                {
                    SetClientCredentials(true);
                }
                else
                {
                    SetTrayTooltip(state);
                    SetState(state);

                    await Task.Delay(PollInterval);
                }
            }
        }