Hi ABK,
Let’s confirm some information:
1. Which Office 365 plan are you using?
2. Are you using Office 365 pre-upgrade or Office 365 after upgrade?
You can also have a look at this article: Am I using Office 365 after the service upgrade?
3. Could you please capture a screen shot of your Admin portal? Including the notification “you received 4500 mail……”.
If you’re using Office 365 after upgrade, let’s discuss this issue as the following:
1. Have you ever configured any specific settings in Exchange Admin Center (EAC)?
2. You can try to check the email messages that are marked as spam in quarantine.
You can view a list of all messages at once or search for specific messages by specifying filter criteria via the quarantine interface. After identifying the specific message, you can see a summary about the message in the right pane.
Moreover, you can also release the messages from Quarantine.
Thanks,
Young Yang