Hi @shicmao ,
Based on my below test, i can change the encoding of the .txt file when i try to save the file:
1.Open an email>click File>Save As>beside Save as type, choose Text Only(*.txt)>type the file name>Click Save.
2.In the pop-up window, click NO.
3.Click Other encoding: choose the encoding.
Please check if you tried the same steps.
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