Use OSX Mail from Panther (10.3) with iCloud email address

The problem is likely that your email service does not support older versions of TLS (aka SSL, aka HTTPS), and Panther does not support newer versions of TLS. This creates an impasse where neither can talk to each other.

To fix this problem, you need an interpreter—a piece of software which sits between you and the server, intercepts your traffic, and modifies it to be compatible. The technical term for this (which you've probably heard somewhere) is a "proxy server", and more specifically a "MitM" or "man-in-the-middle" proxy which decrypts and re-encrypts your traffic before sending it on its way.

For Macs running OS X 10.6 – 10.13, I created an easy-to-install package which should be simple to set up: https://jonathanalland.com/legacy-mac-proxy.html.

Unfortunately, the fact that you're on Panther makes things more difficult. I have been unable to make Squid 4 work reliably on OS X 10.5 and below.

One alternative is to set up Crypto Ancienne, which is said to be compatible with even OS X 10.0 and NeXTSTEP. Note that because Crypto Ancienne does not perform certificate verification, it cannot offer the security guarantees that usually come with https connections.

A second alternative is to run a proxy server on a different computer, and connect to it from your Mac running Panther. For instance, you could install my package on a newer Mac (but not too new), uncomment the line in squid.conf to allow non-localhost connections, and add the newer Mac's IP address to your Panther machine. Alternately, install Squid 4 on a Raspberry Pi (ensure the compile time option --enable-ssl-crtd is enabled), and reuse my squid.conf file, adjusting the paths as needed.

Good luck!

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.