It might happen though, that Mail does not offer the specific character encoding the user is looking for. In order for Mail to expand the list of possible character encodings, quit Mail and then go to System Preferences -> International -> Language tab, and then click the Edit... button. In this dialog, check the specific language that the missing character encoding is intended for. Click OK, quit System Preferences, and open Mail again -- the new character encoding options will appear in the drop down list. For example, in order to be able to choose Shift-JIS as a character encoding in Apple's Mail, add Japanese language to the International preference panel.
This hint is not actually mine, but was gently revealed by macosxhints' forum poster "bedouin" in this thread. So thanks bedouin!

