Unable to email clients using Symantec email security

Hi there,

   For the last week now we have a client who is unable to email anyone that uses Symantec Email Security. They constantly get the below message -:

  They use Office 365 and all DNS is correct for SPF and DKIM. They do not show as blacklisted anywhere and the majority of emails have no attachments. Strangley though if they email from the Office365 web portal the email sometimes gets through. 

 I submitted several emails last week and the week before to be checked for false positives but have had no response. 

 This is causing major disruption for the business as they are finding that more and more suppliers use theis sytem for email filtering. They have had 1 customer whitelist them which allowed email through but they do not want to have to do this with dozens of other clients.

 Any help would be appreciated

 Kind regards,