一封英文信的格式怎樣寫?

睇你係formal 定informal

formal
Dear Mr.XXX,
I am writing to ......
First,.....
Second,.....
At last,......
I am looking forward to your reply. Thank you for your attention.
Yours sincerely,
XXXX

informal
Dear Peter,
Hi, how are you?
.........
Best wishes,
Mary