There's good news for privacy-minded Mac users; Apple has improved the quality of iCloud's email security. All emails sent or received via iCloud email is now encrypted, even when sent to a third-party email service. Previously only emails sent between iCloud users was encrypted by Apple.
While the company hasn't announced the changes, you can the evidence on Google's transparency report website. Transparency Report shows what percentage of emails sent between Gmail and other providers are encrypted in transit. We'll keep you updated if Apple releases a statement about the changes.