Norton disabling Incredimail

Why would Norton suddenly identify Incredimail as a threat?

I updated Incredimail to version 2.5 and within a few minutes Norton said it needed to restart the computer to remove threats.

I rebooted and found that Incredimail then did not work.

Norton had quarantiened most of the DLL files needed by Incredimail so I had to restore all these files from quarantine one by one to get Incredimail working.

Why did this happen?