Thank you for your suggestion.
I resolve this issue by using a custom filter as below
Fortunately, the mails have ‘X-Original-MAILFROM’ tag whose value is ‘outgoing@kiise.or.kr’.
So the filter can catch the mails with this condition.
Still, I think the reason for this issue is the encoding problem, but I am not sure.
Thank you for your replies again.
Yours sincerely