The difference is whether the message contains a correct Content-type: header or not. For example:
Content-type: text/plain; charset=iso-8859-1This says that the message is in the character set iso-8859-1, also known as Latin-1 or “western”. The usual charset used for Traditional Chinese is Big5. The problem is that a lot of software does not add in a Content-type header at all, or adds an incorrect one. The latter is a common problem if you usually use English but occasionally want to send something in Chinese. Technically a message without a Content-type header SHOULD be interpreted as being ASCII only. But because there is so much broken software around, most mailers will default to treating messages as defaulting to a particular charset, and usually pick the default default as whatever charset you normally use.
So for most English speakers and other western europeans, that’s usually iso-8859-1, and for most Tradtional Chinese users, it is Big5. This all works if you stick to one language. Someone sending out a message in Big5 to other Big5 users will have it display in Chinese because all the other Traditional Chinese users will default to Big5 encoding. This has the side effect that they will have no idea their software is broken and assume that it is your problem.
Most mail readers have several settings for default character sets. You could change your default to Big5 and still read most english emails correctly, but you would lose any accented characters from other western european languages. Also some products use funny versions of the quote characters that would also cause difficulty. Some mail programs like Mozilla allow you to set a per-folder default, so you could sort mail from dodgy sources to that folder and be able to read it properly. Or just change the setting each time you come across a broken message (View/Character Encoding on Mozilla based readers).