I'm afraid I don't know much about emailing Chinese on Windows. If you do, send it along to me and I'll stick it in here.
If you have the Chinese Language Kit installed, you can already send and receive email in Chinese, and sometimes it will even work. There are two keys to successful Chinese email. It used to be that a major problem was mail being routed through servers somewhere on the globe that weren't "8-bit clean," which means that they stripped crucial information from the message. This doesn't seem to be much of a problem anymore.
The remaining issue is in the mail headers: the stuff that Eudora mostly hides from you. The problem is that Chinese characters contain "high-Ascii" symbols (like © and ®) and the internet and the Mac code these symbols differently. If these symbols are part of a GB-encoded simplified Chinese word, they must be left intact when sent to someone else; if they are actually a copyright symbol (or whatever), they go through a process of conversion. This problem is solved by labeling the character set that you are sending. This is accomplished in mac Eudora through the use of a Transliteration Table, a small file that you simply drop into the folder containing your Eudora program. You then set incoming and outgoing messages to be labeled with the appropriate character set. It's very simple, and the documentation that comes with the file is quite straightforward. More information, including the files, is available here; the files you need are also available here.
If you are sending and receiving mail with someone who always puts the proper character set headers in their mail, you should be all set. Well, not quite. In Eudora's Settings, you need to make sure that "fix curly quotes" and "word wrap" are unchecked. Since curly quotes are among the high-ascii symbols that make up Chinese encodings, if they are "fixed," your Chinese will turn to garbage. Also, word wrap will wrap after a certain number of ascii characters, even if it is in the middle of a Chinese character (which takes two ascii symbols). So don't do it.
One more problem: what if someone sends you a message containing Chinese that doesn't have the proper character set header? Then Eudora won't know to treat it as Chinese, and will convert all the high-ascii symbols, giving you garbage. Luckily the Tables mentioned above give you an additional capacity. After they've been installed, you can select the text of a message and choose "repair ISO Latin-1," which will undo the conversion (which had been based on the assumption that the incoming message was European--ISO-Latin-1--rather than Chinese).
One more thing: remember that in most cases you'll need to change the font of incoming messages: click on the checkmark that allows you to edit messages, then select all, then choose the new font. Voila.