IT specialist Bob Gendler found that macOS Mail was storing encrypted emails in plain text. He first notified Apple on July 29, but only got a temporary fix from the company 99 days later on November 5.
The main thing I discovered was that the snippets.db database file in the Suggestions folder stored my emails. And on top of that, I found that it stored my S/MIME encrypted emails completely UNENCRYPTED. Even with Siri disabled on the Mac, it *still* stores unencrypted messages in this database!
Mr. Gendler shard a fix in his blog post.
Check It Out: macOS Mail Stores Encrypted Emails in Plain Text