Hi @john john ,
As per my test, I can also reproduce this scenario on my end. Unfortunate though it is, I'm afraid that's by design afaik, no way to embed/iframe from domain "outlook.office.com" even if you have added it in HTML field security.
In this case, I think there is an alternative:
- Create a SharePoint calendar
- connect that calendar to outlook
- Copy the old outlook calendar information to the newly connected SharePoint Calendar. Take a reference to the video in this link: Import an Outlook Calendar to SharePoint
- Embed the SharePoint calendar page to the SharePoint modern page using the Embed web part
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