Hello CGL21,
Thank you for posting this case in the Microsoft Community!
Based on your description, you want to confirm if this is a safe Microsoft 365 email CATCH-ALL solution. I would like to cooperate with you to work on this case.
To clarify this case, are you using a Microsoft 365 E5 developer subscription to set up a catch-all mailbox from your side? To explain further, the catch-all mailbox is also known as full receiving email, it redirects emails sent to email servers that do not have accounts to a specific email address, rather than simply returning the message. Under normal circumstances, if an email is sent to an account that does not exist on the server, it will be returned directly to the sender. E5 developers can subscribe to only 25 users and use the catch-all function to achieve unlimited alias email receiving.
As checked this YouTube link with the video watched, it seems that this is from our Microsoft Partner who shares catch-all deployment guidance with public releases. It is stable and safe as an available feature in Microsoft 365 services. You may check this Microsoft article as a reference guide you how to set up a catch-all mailbox which is also supported in Microsoft 365 for Exchange Online: Configure a Catch-All Mailbox
If you need further assistance, please feel free to let me know, and I will keep assisting you.
Your understanding and patience will be appreciated.
Thank you!
