I recommend opening up a support case on this - I recently saw this happen with a customer due to an incorrect firewall setting that was stripping out the webmail provider's IP address.  These web emails ended up looking like they were coming from local IPs that should never be sending email, and that's why they were getting flagged as Bad Senders.  The fix was to the firewall configuration to ensure that the emails were being reviewed by Symantec Brightmail Gateway with the full set of IP addresses in the headers.

Just saw the note that you already have opened a support case - if you send me a dm with the case number, I'll circle back with the support team.

Our clients use a newsletter system to deliver multiple direct emails Symantec/MessageLabs/BrightMail (I assume they're all the same) customers which, all of a sudden, are being blocked.  We are not spammers, and all emails are sent to clients directly requesting the subscriptions. We are not on on the SGBS list.  furthermore, we've checked our server IP on using this tool http://ipremoval.sms.symantec.com/lookup/ and all other blacklists.

In short - our client emails are being blocked illegitimately (to Symantec's clients and ours) detriment.

I posted an email query with all the details 20 hours ago to support@ - no response yet.

Can anyone provide any advice about stopping Symantec triggering these false positives?

The end users of the SymantecAnti Spam customers should have the ability to submit false positives if they feel the messages are being blocked incorrectly. There are many reasons that email messages could be getting blocked, including custom filters put in place by the customers.

There are unwated mail categories in SMG: -

Marketing mail

Email messages that contain commercial or fund-raising
messages, that may have been requested by the user.
These messages often do not include a functional opt-out
facility.
 

Newsletter

Email messages that include content on specific topics,
on a known periodic basis, often weekly or monthly. The
user may have requested to receive these publications.
A functional opt-out facility is generally available.
 

Suspicious URLs are URLs that may deliver spam or
malware payloads. Suspicious URLs include free hosting
sites, URL shortening services, and URL redirecting
services. Symantec Messaging Gateway can filter against
email messages that contain one or more suspicious
URLs.

Such mail are not classifed as spam, however SMG gives ability to the administrator to set policies to deal with such mails including blocking.

Make sure your customers have not blocked the newsletter category.