Hi,
a user from the Forum I manage reported the following problem with the summary email sent automatically:
Apparently Microsoft Defender for Office 365 classifies the site as malicious.
Is there anything that can be done about this problem?
thank you so much
Hi Michele. 
If we follow the error message the user received, we find:
What to do when you are blocked from a site, and believe the result is mistaken
If you’ve attempted to go to a website and been shown a page that says that the site was blocked or might be malicious, that means that your organization’s policy has blocked that site. These policies can apply to large categories of sites so it’s possible that the site was inadvertently caught - what we call a “false positive”.
If you believe that the site you were trying to connect to is safe, you should contact your organization’s IT administrator or help desk and ask them to update their policy to allow this site.
It appears that user’s email is having a policy applied that can be changed at the organizational level. I’d recommend having the user follow up with their org. 