You can always see why a message was blocked either via the Classification field in the Log Search pages, or by looking at the Evidence line in the Raw view of the message in the Quarantine. In addition, if you have the Features preview enabled, you can find additional details in the Log Search (Preview). You can use the customize drop down list before running the search and have the fields: Main class, Sub class and Extra class returned. These fields will contain more information about the rejection reason.
You can then compare that information with the one we have available in our Classification article here.
Usually, Releasing and Training the messages, action available directly in the Log Search, will make adjustments to our various databases and will resolve the issue.
This applies for both Incoming and Outgoing products. However, as the Hosted Cloud clients do not have access to the messages in the Outgoing Spam Quarantine, they may need to open a support ticket from either the client area or from an authorised email address.