by letter You will receive an acknowledgement by letter.
get/receive a letter (from sb) I got a letter from my lawyer this morning.
deliver/post/send a letter I sent a letter about the problem to HQ last week.
draft/write/sign a letter The letter was signed by the managing director.
reply to/acknowledge/answer a letter I wrote to the company but they did not even acknowledge my letter.
a confirmation/rejection/warning letter You will receive a confirmation letter within 10 working days.
an accompanying/a covering letter Please find enclosed my CV and a covering letter.
a formal/an informal letter He wrote a formal letter of resignation.
The letter was addressed to John Clarke.
a letter dated 3rd March 2010