I apologize for not replying sooner, but I have been very busy these last few weeks.
很抱歉没能及早回信，但是过去几周实在太忙。
Thank you for getting in touch with us about XXXX. (Less formal, more friendly)
非常感谢你关于XXXX联系我们。(不是很正式,更友好)
Thank you for contacting us regarding XXXX. (More formal)
非常感谢您因关于XXXX的事接洽我们(更正式)
With reference to your email of [date], I would like to bring the following to your attention.
关于你(时间)寄来的邮件，我有几点想跟你说的。
As a follow-up to our phone call this morning, I would like summarise the key issues.
关于你早上的电话，我想总结下关键几点。
Phrases best avoided:
请最好避免：
I hope this email finds you well.
我希望这封邮件能被你看到。
Please be advised as follows.
请按照如下建议。
This email concerns…
这封邮件是关于……
