An October 14 report says that Office 365 Message Encryption shouldn’t be used because its encryption scheme might reveal email content. Well, that might be the case if an attacker can hijack connectivity from Office 365 to another email service. But the relatively low levels of OME usage and the difficulty of acquiring enough email to understand message structure makes this a less than practical attack in the wild.
Office 365 Message Encryption protection is not available for messages sent to dynamic distribution groups. It’s all to do with rights management licensing. However, if you need to protect messages sent to dynamic distribution groups, for instance to make sure that confidential messages are inaccessible to external recipients use a sensitivity label instead and assign the special tenant-wide permission to recipients.