I have had this problem for a while.
Background: NIS, Thunderbird for Email, Window 10
I understand that NIS does not support Thunderbird as an Email client, but NIS does somehow intercept the data stream coming in via POP and modifies the subject line of any message it considers to be spam by prepending "[Norton AntiSpam]" to the subject.
To make up for the fact that NIS doesn't support Thunderbird, I added a Thunderbird filter that looks for messages with "Norton AntiSpam" in their subject lines and moves such messages to Trash. To deal with erroneous spam classifications, I have added several domains and email addresses to the NIS Allowed List (see attachment). This all works well for the other entries in the Allowed List, but not for messages from Edward Jones, which still get flagged as spam. First I added the domain (edwardjones.com) to Allowed List, and when that didn't work, I added the specific email address (removed). As of this morning, messages from this Email address are still getting flagged. Excerpt from the source of such a message:
Reply-To: "Edward Jones" <removed>
Date: Fri, 01 Jul 2016 01:33:42 EDT
From: Edward Jones <removed>
Subject: [Norton AntiSpam]Your Trade Confirmation Is Ready
I had a long, unsatisfactory Chat session with <removed> this morning (case number <removed>). After using LogMeIn to check everything, her bottom line was that, since Thunderbird is not a supported browser, the problem was with Thunderbird or Earthlink (my ISP). Small excerpt:
5:32 AM : By tag, I mean "[Norton AntiSpam]" - do you mean something different?
5:33 AM Agent: As I have said. It will only shows as the subject line but is not the one tagging the email as SPAM
5:34 AM : I ask again, if NIS is not tagging the Subject line, then who is?
She never answered this question, but obviously NIS is the only actor that would add "Norton AntiSpam" to the subject line. The bug is that NIS, in doing so for these messages, is not following its own rules.
[Edit: Removed Case Number to confirm with Participation Guidelines & Terms of Service ]