https://www.bleepingcomputer.com/news/security/microsoft-to-make-office-365-encrypted-emails-look-less-spammy/

Microsoft is currently working on enhancing the way emails sent using the Office 365 Message Encryption (OME) service are seen by mail servers so that they are less likely to be marked as spam and sent to the Trash folder.

OME is built on Microsoft Azure Rights Management (Azure RMS), part of Azure Information Protection, and it allows Office 365 customers to send and receive encrypted email messages using Outlook.com, Yahoo!, Gmail, and several other email services using encryption, identity, and authorization policies.