How many letters did the Corinthians write to Paul?

It is apparent from 1 Cor. 16:8 that Paul is writing from Ephesus ("I will stay in Ephesus until Pentecost"). He is responding to a number of specific issues in his letter (most certainly not his first letter). How did he know this detailed information? The answer: by both oral and written accounts. Regarding the latter, for instance, 1 Cor. 7:1 has "Now concerning the matters about which you wrote..." We then learn that Paul received written information specifically about marriage problems. When asking the question: did the Corinthians write to Paul? We can answer with some confidence that the letter writing was reciprocal. Unfortunately, only two letters survive and they are from Paul.