Emails are adding giberish

when replying to an email, the emails are garbled with chinese charachters.
Details of what’s going on:
After it was switched to GB2312, I did below testing in Outlook and Webmail . All have messy code.(see screen shots)

The problem is (my current settings is Chinese GB2312 and HTML, actually we did try all kinds of encodings ), when we sent out an email to external FIRST TIME on their Clients shows normally and they can read Chinese in emails,

but after they reply us or forward some Chinese emails to us the original emails have garbled issue. The problem like the original messages have been re-encoding once???
Anyone have any ideas?