Outlook for iOS and Android now support marking and encryption of email with Office 365 Sensitivity Labels. Sensitivity labels can now be applied through Office ProPlus, OWA, and Outlook mobile. All that really remains to achieve full coverage for sensitivity labels across Office 365 are the Office Online and SharePoint/OneDrive browser interfaces. In other news, Outlook Mobile also supports S/MIME.
Microsoft Cloud App Security (MCAS) can integrate with Azure Information Protection to allow automated policy-driven application of Office 365 sensitivity labels to Office documents and PDFs. You can depend on users to apply labels manually as they create documents, but it’s easy for humans to forget to add protection where a computer won’t. You’ll pay extra for MCAS, but it could be worthwhile.