@CD001
When you insert a YT video in the Reply box, it's actually added as an iFrame. Most email providers do not allow iFrames for security reasons. So you have three options:
1.) Insert a link to the YT video instead of putting it in an iFrame
2.) Find and use an email provider that allows iFrames
3.) Have the User login to the Client Portal to view the Ticket Thread (which will properly display the iFrame)
Unfortunately, there is nothing we can do about this as it's the email provider stripping the iFrame out when it reaches the Inbox. I did some testing and can confirm that the iFrame is in the content when the email is sent out from the system so nothing to do here as far as our code goes.
Cheers.