As a Western Email Developer I was asked to develop emails in Chinese Simplified, Chinese Traditional, Japanese and Korean (CJK). After a lot of research, testing and actual sends here is my guide.
First of all we need to declare English target fonts before Chinese target fonts, because English language fonts do not contain the glyphs for Chinese characters, but Chinese fonts do contain a-z characters. This means if you declare the Chinese fonts first, any English-language computer that has the standard Chinese font faces installed will display English characters using Chinese fonts (Example: Western installs of the Operating Systems) . This means English characters will be rendered in the first font and Chinese characters will be displayed using the fall-back Chinese fonts. Even if the Email is entirely in Chinese, English character will pop-up on occasion, so it's good to declare this way. The same theory goes for Japanese and Korean declarations.
Fonts for Chinese Simplified and Chinese Traditional:
Windows
宋体, SimSun
OS X
华文仿宋, STFangsong
The above is my tried and tested fonts, other fonts available are:
Windows
黑体：SimHei
新宋体：NSimSun
仿宋：FangSong
楷体：KaiTi
仿宋_GB2312：FangSong_GB2312
楷体_GB2312：KaiTi_GB2312
微软雅黑体：Microsoft YaHei
OS X
冬青黑体: Hiragino Sans GB
华文细黑：STHeiti Light [STXihei]
华文黑体：STHeiti
华文楷体：STKaiti
华文宋体：STSong
What this looks like in the declaration:
font-family: Arial, 宋体, SimSun, 华文仿宋, STFangsong, sans-serif;
Fonts for Japanese:
Windows
メイリオ, Meiryo
ＭＳ Ｐゴシック, MS PGothic
OX X
ヒラギノ角ゴ Pro W3, Hiragino Kaku Gothic Pro
What this looks like in the declaration:
font-family: Arial, メイリオ, Meiryo, ＭＳ Ｐゴシック, MS PGothic, ヒラギノ角ゴ Pro W3, Hiragino Kaku Gothic Pro, sans-serif;
Fonts for Korean
Windows
돋움, Dotum
OS X
굴림, Gulim
What this looks like in the declaration:
font-family: Arial, 돋움, Dotum, 굴림, Gulim, sans-serif;
It's best practice to use both the Local and English spelling of the fonts along with the PC and Mac declaration.
Attached below is a text document with declarations for easy reference.
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