Explanation:
The reason for writing the letter comes always in the first.
The reader should first know, why the writer has written.
If the purpose of the letter is mentioned in the last, the reader wouldn't be interested and it is not the best practice also.
The last part should comprise of requesting, or re-insisting the purpose of the letter.
The middle paragraph should go in deeper by giving some proof or explaining the process in a descriptive manner.
