With reference to …
Further to …
With regard to …
I am writing in connection with …
注：引入主题的句子可以被写在信件的开头，但with regard to 不能用于信件的开头。
环境：(How would you start a letter about each of the following?)
an invoice (No. 679) for a photocopier
With reference to Invoice No. 679, we regret to tell you that the product arrived in bad condition.
a meeting you had with the addressee on Jan. 16th
Further to the meeting on 16 January, I am delighted to tell you that we are now able to give you a special offer for our Peach Series computer.
an advertisement in the times newspaper
I am writing in connection with the advertisement in the times newspaper yesterday.
an application for a post as secretary in your company
I am writing in connection with your application for a post as secretary in our company.
a fax order for six computers that you received today
with reference to your fax order for six computer, I regret to tell you that they are out of stock at present.