iCloud mail doesn't work anymore with macOS 10.9.5
since feb 18 2025
[Re-Titled by Moderator]
Question marked as
System requirements for iCloud - Apple Support
According to this article – which currently has "Published Date: February 12, 2025," Cloud Mail no longer supports any version of macOS earlier than macOS 10.12 (Sierra). For that matter no iCloud features support anything before Sierra.
Looks like Apple decided to retire support for older OSes.