If you don’t know the name of the person you are writing to, start business emails with either of the following:
如果你不知道郵件是寫給誰的，商務信函可以用下面任意一種方式開頭：

Dear Sir,/Dear Madam,
尊敬的先生,/尊敬的女士,

Dear Madame, is wrong.
“親愛的夫人”這種寫法是錯誤的

If you don’t know whether you’re writing to a man or a woman use:
如果你不知道是寫給先生還是女士，可以寫：

Dear Sir or Madam,
尊敬的先生或女士，

It is not usual to start an email To whom it may concern. This should be reserved for letters of reference or similar communications when the recipient is an unknown third party.
以“致相關人士”開頭并不常見。這種用法一般用在推薦信或與其相似的交流中，收件人是不了解的第三方。

It is always better to use somebody’s name if you know it. If it is the first time you are writing to someone, use either of the following:
如果知道對方姓名，郵件中寫上總是好的。如果這是你第一次寫信給某人，可以用下面任意一種方式開頭：

For men: Dear Mr XXXX,
寫給男士：尊敬的XXX先生

For women: Dear Ms XXXX,
寫給女士：尊敬的XXX女士

Once you get to know someone, i.e. after exchanging one or two emails or if you meet them in person, it is usually OK to use their first name.
一旦你認識了對方，比如在往來一兩封郵件或見過面以后，可以在郵件中稱呼名字。

Use Dear Sirs, if you are writing to more than one person even if the group of people includes women.
如果你寫郵件給兩個或以上的人，即使這些人里有女士，請寫“尊敬的先生們”

Dear Ladies and Gentlemen, is wrong. “Ladies and Gentleman” is only used in formal speech.
"尊敬的女士們先生們”是錯誤的，這種用法只用在正式演講中。

The word Dear may be omitted in less formal emails. Instead, you may just open with the person’s first name/people’s first names.
“dear”在不是特別正式的郵件中可以省去。你可以直接以對方（個人或群組）的名字開頭。

Here are some example opening sentences for emails:
下面有一些郵件開頭的例句：

I hope you are well.
我希望你一切都好。

I hope you enjoyed your holiday and are finding it easy to settle back in to work.
我希望你已經好好享受假期并且很快重新投入工作。

Thank you very much for your email. I am glad to hear that you and your family are well.
非常謝謝你的郵件，我很高興聽到你和你的家人都很安康。

Thank you for your prompt reply.
感謝您的及時回復。

I apologize for not replying sooner, but I have been very busy these last few weeks.
很抱歉沒能及早回信，但是過去幾周實在太忙。

Thank you for getting in touch with us about XXXX. (Less formal, more friendly)
非常感謝你關于XXXX聯系我們。（不是很正式,更友好）

Thank you for contacting us regarding XXXX. (More formal)
非常感謝您因關于XXXX的事接洽我們（更正式）

With reference to your email of [date], I would like to bring the following to your attention.
關于你（時間）寄來的郵件，我有幾點想跟你說的。

As a follow-up to our phone call this morning, I would like summarise the key issues.
關于你早上的電話，我想總結下關鍵幾點。

Phrases best avoided:
請最好避免：

I hope this email finds you well.
我希望這封郵件能被你看到。

Please be advised as follows.
請按照如下建議。

This email concerns…
這封郵件是關于……