Apple is investigating an issue raised by a Mac specialist discovered to be storing emails that are supposed to be S/MIME-encrypted as readable files. Bob Gendler discovered the problem while he was investigating how Mac OS and Siri suggest contacts and information to users. He discovered that even with Siri disabled, the OS still collects and stores data for Siri, in effect, storing encrypted emails without encryption in a database. Apple has acknowledged that they received his information and said the company is investigating the issue.
Source: https://threatpost.com/encrypted-emails-on-macos-found-stored-in-unprotected-way/150065/