The encryption method of e-mails in Microsoft's Office 365 package does not appear to be completely watertight. Patterns can be recognized in the encrypted data, which means that the content of e-mails may still be partially traceable.
Microsoft's OME (Office 365 Message Encryption) uses the so-called ECB block encryption, in which data is encrypted in blocks. The block size is always the same, so that two identical pieces of text still look the same after encryption.
As a result, when comparing larger amounts of encrypted text (or emails in this case), you will discover patterns, and thus be able to decipher the content again.
The vulnerability was already discovered in January by a researcher from security company WithSecure. Microsoft acknowledged the flaw and rewarded the researcher, but still hasn't released a patch. After repeated inquiries by WithSecure, Microsoft does not consider the risk large enough to make adjustments.