NIS Antispam Plugin With Outlook 365 Results in Endless Password Prompts

Recently I started having a problem with Outlook 365; almost every time I start the program, I am prompted for email account passwords that used to be saved. I have found that removing the Norton Antispam plugin in Outlook solves the problem. Unfortunately, the antispam plugin appears to reinstall itself every time I reboot. In NIS Settting/Network/Message Protection, I have disabled Antispam completely - yet the plugin still reinstalls itself. Help!

How can I either get Outlook to remember my email passwords again OR how can I permanently remove the Norton Antispam plugin so that it does not reinstall every time I reboot?

Thanks!