The server committed a protocol violation The server response was:

public class YlpSmtpClient
    {
        readonly string _fromEmailAddress;
        readonly string _host;
        readonly string _username;
        readonly string _password;
        public YlpSmtpClient(string host, string username, string password, string fromEmailAddress)
        {
            _host = host;
            _username = username;
            _password = password;
            _fromEmailAddress = fromEmailAddress;
        }

        public delegate YlpSmtpClient CreateSmtpClient();
        protected YlpSmtpClient() { }

        public virtual async Task SendMailAsync(string subject, string body, string toEmailAddress, bool isBodyHtml = false)
        {
            try
            {
                using (var client = new SmtpClient(_host) { Credentials = new NetworkCredential(_username, _password) , Port = 587 })
                {
                    using (MailMessage message = MakeMessage(_fromEmailAddress, subject, body, toEmailAddress, isBodyHtml))
                    {
                        //process exits when calling this line
                        await client.SendMailAsync(message);
                    }
                }
            }
            catch (System.Exception ex)
            {

            }
        }

        MailMessage MakeMessage(string fromEmailAddress, string subject, string body, string toEmailAddress, bool isBodyHtml)
        {
            var message = new MailMessage(fromEmailAddress, toEmailAddress, subject, body);
            message.IsBodyHtml = isBodyHtml;
            return message;
        }
    }

public class EmailNotifier<TNotificationEntity> : INotifier<TNotificationEntity>
        where TNotificationEntity : NotificationEntity
    {
        private readonly INotificationTemplater<TNotificationEntity, string> _notificationTemplater;
        private readonly YlpSmtpClient _smtpClient;
        private readonly INotificationsService<TNotificationEntity> _notificationsService;

        public EmailNotifier(
            INotificationTemplater<TNotificationEntity, string> notificationTemplater,
            YlpSmtpClient smtpClient,
            INotificationsService<TNotificationEntity> notificationsService)
        {
            _notificationTemplater = notificationTemplater;
            _smtpClient = smtpClient;
            _notificationsService = notificationsService;
        }

        public async Task SendAsync(TNotificationEntity notification)
        {
            if (notification.EmailIsOn)
            {
                var templatedMessageBody = await _notificationTemplater
                    .WrapInTemplateAsync(notification, NotificationType.Email);

                await _smtpClient.SendMailAsync(notification.Subject, templatedMessageBody, notification.EmailAddress);
                await _notificationsService.MarkNotificationAsSentAsync(notification, NotificationType.Email);
            }
        }
    }