Failure sending mail.

[kulame]

I am trying to send an email to Feishu email server. my code is

builder.Services.AddFluentEmail(smtpUsername).AddRazorRenderer().AddSmtpSender(new SmtpClient()
{
    Host = smtpHost,
    EnableSsl = true,
    DeliveryMethod = SmtpDeliveryMethod.Network,
    Port = smtpPort,
    UseDefaultCredentials = true,
    Credentials = new NetworkCredential(smtpUsername, smtpPassword)
});
        Task.Run(async () =>
            {

                using (IServiceScope scope = _factory.CreateScope())
                {
                    _logger.LogInformation($"Sending email to {user.Name}");
                    var _fluentEmail = scope.ServiceProvider.GetRequiredService<IFluentEmail>();
                    _logger.LogInformation("got email service");
                    try
                    {
                        await _fluentEmail.To(user.Email)
         .Body("The body").SendAsync();
                        _logger.LogInformation("Email sent");
                    }
                    catch (Exception ex)
                    {
                        _logger.LogInformation(ex.Message);
                        _logger.LogError(ex.StackTrace);
                        _logger.LogError(ex.)

                    }
                }
            });

it raise error

info: SocialApi.Services.TaskService[0]
      Failure sending mail.
fail: SocialApi.Services.TaskService[0]
         at FluentEmail.Smtp.SendMailEx.SendMailExImplAsync(SmtpClient client, MailMessage message, CancellationToken token)
         at FluentEmail.Smtp.SmtpSender.SendAsync(IFluentEmail email, Nullable`1 token)
         at SocialApi.Services.TaskService.<>c__DisplayClass4_0.<<SendEmail>b__0>d.MoveNext() in /srv/social/SocialApi/src/Services/TaskService.cs:line 44

what's wrong with this code?