問候語 - greeting

如果問候別人關於 她/他 的心情, 用regard比較好D,
e.g. regard to -> it means you pay attention to or thought and care for somebody
雖然我們用"best regards"用於信的結尾, 但用於標題可表現你出於關注
e.g. "a deeply regard about you", "send a message with regard", "with kindly regards"

greeting通常用於"a message of good wishes for somebody" or "you say something to greet 祝賀 somebody"

Greeting (n) 歡迎 近義詞：Welcome
看情況，這個情況下用來做標題就不太貼切。
其次，英文寫信好像不用標題。

A friendly greeting.
Good wishes for your health.

How are you?

How are you?可用在負面的情況.