I don't see anything wrong with the defaults command you've used here. And you're using the correct key for the Outlook version you specified. (This specific key use to have a typo, but was corrected a while back. It use to be AutomaitcallyDownloadExternalContent. Oy!)
I was able to set this using your command in Terminal. It works, but you must do it only while Outlook is quit. If you're testing while Outlook is running, then it'll override the plist and revert the change as it's saving its settings.
Because you're running this from Jamf Pro, you must also script this to run for each user. Otherwise, it'll run for the root account and you won't see any change in behavior.
I've tested this script minimally, but you should be able to add it to a policy in Jamf Pro to set the key/value for every user on a Mac. I recommend running it at Login to ensure Outlook isn't running for anyone.
#!/bin/bash
# create a list of all users with UIDs over 500
userList=$( /usr/bin/dscl . list /Users UniqueID | awk '$2 > 500 { print $1 }' )
# create or modify existing com.microsoft.Outlook setting
for aUser in $userList
do
# run the defaults command as a user
su "$aUser" -c '/usr/bin/defaults write com.microsoft.Outlook AutomaticallyDownloadExternalContent -int 2'
# test whether the command was successful and report
if [ $? = 0 ] ; then
echo "Setting Outlook key 'AutomaticallyDownloadExternalContent' to '2 = In all messages' for user $aUser."
else
echo "Failed setting Outlook key 'AutomaticallyDownloadExternalContent' to '2 = In all messages' for user $aUser."
fi
done
exit 0
