This functionality is important for 10 billion of people. Please consider it.
In Chinese environment, we always receive email in different charset (zh_tw utf-8, zh_TW, zh_CN utf-8, zh_CN, zh_tw_HK utf-8). When we reply message, the reply email charset is set to user setup only. Other charset which is not same as the user setup will display abnormally in both user interface and reply email content.
If reading and replying email was set to the charset of receiving email. It would be very helpful.