Outgoing mail is considered spam

Hi,

I have a user in our tenant who sends emails to multiple people at one time. The maximum number is 200 recipients at a time per day. This concerns 1 email with, for example, 200 recipients. Now, after the email has been sent, this user is marked as Spam and the account is blocked. When I then look at the reason, it says Domain reputation.

The user also remains within Microsoft's sent limits.
How can I find out or where can I within O365 what the exact reason is why this user is blocked and the email is considered spam.

There are several users who do this and do not receive any notifications.

Can someone help me with this?

Kind regards,

Jacob