OrderPaid.StoreOwnerNotification message template sends emails to store owner but not BCC recipients

I have really been struggling the last couple of days to figure this out. I have correctly configured the email address (a Gmail email address), and the test email successfully sends to a non-gmail account (my personal email). I then set up the OrderPaid.StoreOwnerNotification message template to not only send the order paid email to the owner email, but to send it to my personal email as well as a BCC recipient (the SAME email address I sent the test email to successfully).

This is where the problem starts. The notification email is then successfully sent to the owner (gmail) account, but not to the BCC recipient. I then checked the message queue, and there is nothing waiting to be sent. I then checked the log (system -> log), and an error appears which states the following:

Error sending e-mail. The SMTP server requires a secure connection or the client was not authenticated. The server response was: Authentication required

System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: Authentication required 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 Nop.Services.Messages.EmailSender.SendEmail(EmailAccount emailAccount, String subject, String body, String fromAddress, String fromName, String toAddress, String toName, String replyTo, String replyToName, IEnumerable`1 bcc, IEnumerable`1 cc, String attachmentFilePath, String attachmentFileName, Int32 attachedDownloadId) at Nop.Services.Messages.QueuedMessagesSendTask.Execute()

I have already set up the gmail account such that it lets less secure apps access it, so that's not the issue. Could someone please give me some suggestions to try? Because this is starting to get very frustrating. I NEED these BCC recipients to receive the same email as well, but am having absolutely no luck at the moment.