我非常愉快地回复您1998年8月18日的来函查询。
We are pleased to respond to your inquiry of August 18, 1998.
We are pleased to answer your letter of August 18, 1998.
以下是就您1998年10月21日来信的回函。
We are writing in response to your letter of October 21, 1998.
We are writing with reference to your letter of October 21, 1998.
这是对您1998年10月29日来信的复函。
This refers to your letter of October 29, 1998.
这是对您1998年11月4日来信给予的答复。
This is in response to your letter of November 4, 1998.