You may that Outlook thinks the Quarantine Summary message is junk. This can happen because the message lists details of emails that have been quarantined. Outlook may recognise email addresses or words (Viagra, pharmacy, gambling, etc.) in this summary as potential Spam.
To prevent this from happening in the future:-
- Right-click on the message.
- Choose Junk Email -> Add Sender to Safe Senders List.
Or manually add email@example.com to your Safe Senders List.