432. I am writing to you to complain about...
本句型是典型的投诉信件的开头。complain about sth.是固定搭配，表示“抱怨某事物”，about不能替换为其他介词。
※ I am writing to express my dissatisfaction with your service.
※ I beg to call your attention to the fact that your service is very dissatisfactory.
※ I wish to refer you to an incident which occurred last Friday, calling for some remedial action.
※ I very much regret to have to inform you your service is very dissatisfactory.
※ I am totally disappointed to find that your service is very dissatisfactory.