office365 SMTP sever response 4.3.111

This is NOT a typical project. I just need some advice who understands what the error means. I created a c# program to use office365 SMTP to send email. It works well. However, the following errors happen from time to time randomly.

System.Net Error: 0 : [8940] Exception in SmtpClient#21800467::Send - Error in processing. The server response was: 4.3.111 Temporary server error. Please try again later MRH1 [Hostname=MN2PR17MB3085.namprd17.prod.outlook.com].
at System.Net.Mail.DataStopCommand.CheckResponse(SmtpStatusCode statusCode, String serverResponse)
at System.Net.Mail.DataStopCommand.Send(SmtpConnection conn)
at System.Net.Mail.SmtpConnection.OnClose(Object sender, EventArgs args)
at System.Net.ClosableStream.Close()
at System.Net.Mail.MailWriter.Close()
at System.Net.Mail.SmtpClient.Send(MailMessage message)
System.Net Error: 0 : [8940] Exception in SmtpClient#39501348::Send - Error in processing. The server response was: 4.3.111 Temporary server error. Please try again later MRH1 [Hostname=MN2PR17MB3085.namprd17.prod.outlook.com].
at System.Net.Mail.DataStopCommand.CheckResponse(SmtpStatusCode statusCode, String serverResponse)
at System.Net.Mail.DataStopCommand.Send(SmtpConnection conn)
at System.Net.Mail.SmtpConnection.OnClose(Object sender, EventArgs args)
at System.Net.ClosableStream.Close()
at System.Net.Mail.MailWriter.Close()
at System.Net.Mail.SmtpClient.Send(MailMessage message)
System.Net Error: 0 : [7788] Exception in SmtpClient#14783937::Send - Error in processing. The server response was: 4.3.111 Temporary server error. Please try again later MRH1 [Hostname=MN2PR17MB3085.namprd17.prod.outlook.com].
at System.Net.Mail.DataStopCommand.CheckResponse(SmtpStatusCode statusCode, String serverResponse)
at System.Net.Mail.DataStopCommand.Send(SmtpConnection conn)
at System.Net.Mail.SmtpConnection.OnClose(Object sender, EventArgs args)
at System.Net.ClosableStream.Close()
at System.Net.Mail.MailWriter.Close()
at System.Net.Mail.SmtpClient.Send(MailMessage message)