Sending Emails Error

I am trying to send emails with the code below from localhost using the version 3.3.2 (.Net Core):

public class SingleEmailSenderJob : BackgroundJob<SingleEmailSenderJobArgs>, ITransientDependency
   {
       private readonly IRepository<User, long> _userRepository;
       private readonly IEmailSender _emailSender;

       public SingleEmailSenderJob(IRepository<User, long> userRepository, IEmailSender emailSender)
       {
           _userRepository = userRepository;
           _emailSender = emailSender;
       }

       [UnitOfWork]
       public override void Execute(SingleEmailSenderJobArgs args)
       {
           _emailSender.Send(args.ToEmailAddress, args.Subject, args.Body);
       }
   }

Everything else is pretty standard as per the docs. The Email settings in the Db are:

Abp.Net.Mail.DefaultFromAddress: [e-mail]
Abp.Net.Mail.DefaultFromDisplayName: Core App
Abp.Net.Mail.Smtp.Host: smtp.gmail.com
Abp.Net.Mail.Smtp.Port: 587
Abp.Net.Mail.Smtp.UserName: [e-mail]
Abp.Net.Mail.Smtp.Password: Password123#
Abp.Net.Mail.Smtp.Domain: http://localhost:21021
Abp.Net.Mail.Smtp.EnableSsl: True
Abp.Net.Mail.Smtp.UseDefaultCredentials: False

However, sending fails with this error:

System.Net.Mail.SmtpException occurred
  HResult=0x80131500
  Message=The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at
  Source=<Cannot evaluate the exception source>
  StackTrace:
   at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)
   at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, MailAddress from, Boolean allowUnicode)
   at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception)
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at Abp.Net.Mail.Smtp.SmtpEmailSender.SendEmail(MailMessage mail)
   at Abp.Net.Mail.EmailSenderBase.Send(String to, String subject, String body, Boolean isBodyHtml)
   at CoreLotto.Notifications.SingleEmailSenderJob.Execute(SingleEmailSenderJobArgs args) in C:\Users\user\Desktop\Production Apps\CoreLotto\src\CoreLotto.Core\Notifications\SingleEmailSenderJob.cs:line 27
   at Castle.DynamicProxy.AbstractInvocation.Proceed()
   at Abp.Domain.Uow.UnitOfWorkInterceptor.PerformSyncUow(IInvocation invocation, UnitOfWorkOptions options)
   at Castle.DynamicProxy.AbstractInvocation.Proceed()

Please does anyone know what I am doing wrong?

[coommark]

Ha! Well, I guess the problem was with gmail. I tinkered with security settings on Gmail, whatever it was I did worked. Now it is sending okay using _emailSender.Send(args.ToEmailAddress, args.Subject, args.Body); Let me close this. Thanks @acjh for the patience man. Really digging Abp. Guess for .net development, I am going to always swear by it. Saves lots of work.

[acjh]

Do you know why that is a fix? Does it still work if you comment out the ServicePointManager line?