c# Office365 SMTP Exception received 5.7.1 Client was not authenticated

I am trying to send an email from within a c# app via Office365.

I am receiving the following error:

[test out message sending] System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Client was not authenticated.

My code is simple at the moment
        public static void SendMail()
        {
            SmtpClient client = new SmtpClient("smtp.office365.com", 587);
            client.EnableSsl = true;

            client.Credentials = new System.Net.NetworkCredential("me@bubbles.com", "Pa55w0rd");
            client.Timeout = 5000;
            client.UseDefaultCredentials = false;
            client.DeliveryMethod = SmtpDeliveryMethod.Network;
            
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress("me@bubbles.com");
            mail.To.Add("friend@bubbles.com");
            mail.Subject = "test out message sending";
            mail.Body = "this is my message body";
            mail.IsBodyHtml = true;

            client.SendCompleted += new SendCompletedEventHandler(SendCompletedCallback);

            client.SendAsync(mail,mail);
            Console.WriteLine("Sending Message...");
        }

        private static void SendCompletedCallback(object sender, AsyncCompletedEventArgs e)
        {
            // Get the unique identifier for this asynchronous operation.
            MailMessage msg = (MailMessage)e.UserState;

            if (e.Cancelled)
            {
                Console.WriteLine("[{0}] Send canceled.", msg.Subject);
            }
            if (e.Error != null)
            {
                Console.WriteLine("[{0}] {1}", msg.Subject, e.Error.ToString());
            }
            else
            {
                Console.WriteLine("Message sent.");
            }

        }

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I've looked through lots of posts for this, but solutions recommended do not seem to change my fault.
west-comAsked:
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Mitchell MilliganInformation Technology Network AdministratorCommented:
In office 365, port 587 requires authentication, and to have the sending ip address added as a connector in the panel to trust the incoming message.  Alternatively you can use port 25 for SMTP to office 365 and it should go through without a trust issue.  You can see more about that here: How to Allow a Multi-function Device or Application to Send E-mail through Office 365 Using SMTP
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xav056Commented:
Sign in through OWA and verify your mailbox address from the help icon
see if replacing smtp.office365.com with your actual server name resolves it.
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west-comAuthor Commented:
Many thanks for these two. I've just been dragged off this project for a couple of days. Will try these two options shortly. I wanted to avoid port 25 as many of our clients block port 25 on the router to all but the mail server to prevent viruses emailing from PCs and causing the external IP being Blacklisted.
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west-comAuthor Commented:
Both solutions worked. Thank you
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