I have a web client that sends support emails via Office 365. I have started to get failed send messages with the following
System.Net.Mail.SmtpException: Service not available, closing transmission channel. The server response was: 4.3.2 Service not available
My Office 365 portal does not show any connectivity issues. The web client sends approximately 15 emails concurrently and around 40% are failing. How do I diagnose Office 365 for what may be causing this?