Hi @mara2021 ,
Thank you for posting to Microsoft Community.
Based on your description, I want to check with you if you mean there are some mails went to the user mailbox meanwhile it appeared in quarantine. If yes, it most likely something made the email be split into two copies.
Could you please try the followings?
- Please kindly turn off the Zix filter to see if it makes a difference.
- Also, could you offer your Quarantine policies with me to check if it has an effect on the issue.
- Please run a message trace for the delivered emails. It will show what actions were taken on the message before it reached its final status. You could refer to the steps of Message trace in the new EAC in Exchange Online | Microsoft Learn.
Hope it helps and looking forward to your update.