Shared Mailbox gets blocked

Our users send emails from shared mailboxes and now the boxes are regularly being blocked from sending out email. We use Outlook on the Web. 

Error Message: A high-severity alert has been triggered

Activity: Potentially compromised user account

Details: User << Email ID >> has been restricted from sending messages outside the organization due to potential compromised activity.
The details give a large number of emails being suspected as spam.

When I go to Sent Items, they are all legit emails.

How do I check which outgoing messages are marked as spams?
I suspect it's the number of emails and nothing else that is triggering the event.

Please help. I contacted Microsoft support, but nothing has been suggested to resolve the issue.