根据《朗文当代英语词典》，hello 用于打招呼的主要用法如下：

1. used as a greeting when you see or meet someone 用作看见或遇见某人的招呼语。

2. used when answering the telephone or starting a telephone conversation 用作打电话用的招呼语

3. used when calling to get someone's attention 用于引起他人的注意时的招呼语

4. used when you think someone is not acting sensibly or has said something stupid 用于表示认为别人行为不明智或说话愚蠢

5. [BrE] used to show that you are surprised or confused by something 用于表示惊讶或不解

可见，hello 并没有用于书信开头作招呼语的用法。