Use OSX Mail from Panther (10.3) with iCloud email address
In 2018, Apple, Google, Microsoft, and Mozilla deprecated TLS 1.0 and 1.1 effective March of 2020. TLS 1.2 was created in 2008, and thus postdates Panther.
Unfortunately, this seems to imply that Mac OS X Mail in Panther does not support the security protocols currently required to access iCloud mail servers, or any other modern web server requiring a secure connection.