It appears Nortons has put an add on to Outlook which is no longer available with the free version.
This site shows the switches for outlook to check ad ins are the problem
https://www.outlook-apps.com/load-outlook-no-add-ins/Or you could look for the add in as below
1. Open the Outlook desktop client
2. Go to the File tab.
3. Click Options.
4. In the Add-ins section on the left, choose COM Add-ins and press Go.
5. Check the Add-in you want to remove and press the Remove button at the right
You are looking for Norton Antispam
6. Repeat for additional plugins or press the X at the top right to close the dialogue.