Why doesn't UEM create this registry item?
Add the Office 2013 admx template files to UEM and try again:
Just for reference for anyone who needs to write to this registry, but there isn't an actual ADMX template for it, you can create your own following this simple guide Create Group Policy ADM and ADMX templates – 4sysops and load it into UEM. I had to do this for something very specific related to a plugin & it works.
Thanks, JHBPJF, I would have suggested something similar in case the JunkMailImportLists setting would not have been part of the ADMX 🙂
I like the ADM-to-ADMX route they suggest, BTW. Neither format is fantastic to write in, but ADM has a slight edge...