This is the 6716th most frequent Korean word.
응답한
"응답한" means "responded" or "answered" in English.
응답한 is used as a descriptive action specifying the person who answered.
질문에 응답한 사람이 박사님이었다.
The person who answered the question was the professor.
응답한 is used here to express a past action of responding, negated in the context.
그는 즉각적으로 나에게 응답한 적이 없다.
He has never responded to me immediately.
응답한 is used to describe the action of responding as connected to the email being checked.