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Further to our letter dated 
May I refer to our exchange of correspondence regarding … and particularly to our last letter of ..
I enclose a copy of our previous letter of … just in case the original has gone astray in the post
In receiveing our files it come to our attention that we did not receive a reply to out letter of ..

These are some sentence patterns relevant to letters.Just get yourself a dictionary,and you'll have no difficulty in getting to know their exact meanings. Why not try translating them first? I suppose it will be of some benefit to you.