Mutliple False Spam Positives from O365 Sender

Hi

I'm getting false spam positives from MessageLabs when multiple users send emails from their O365 environment to MessageLabs clients.  It's not all emails, just some and has only started happening in the past 10 days.  This is not happening to users of other email security software/services.

I've checked the SPF and similar configuration on the O365 side and all is fine.  I've also checked blacklists and have sent emails to multiple Spam analysis sites and they've reported back that all is OK.

How do I get Symantec/MessageLabs to investigate this and provide me detail on why these emails are being falsely identified?

Thanks

Greg