we have received your letter of May 16,2002,for which we tha

we have received your letter of May 16,2002,for which we thank you.中的for which相当啥意思.
1for which 怎么用,近义词有.
补充：这是我学商务函电中的一句话.感觉函电语法挺复杂的.跟日常英语不同,难度大点.请问哪位高人对商务英语或函电能指点一二.