Symantec DLP cloud service for Exchange Online?

Third-party DLP solutions are a good idea if you have data in multiple cloud services and want to apply the same rule sets to all.

However, if your data is all in Office 365, I would question why you want to spend the money to use a third-party service when considerable DLP capabilities exist within Office 365 (and are getting better over time). Perhaps a unique feature exists that is not available inside Office 365 that the business absolutely needs and can justify the cost. If so, that's great and it's a good reason to use the third-party service. If not...