[Date]

[Addressee]

Dear [Name]:

We have received your Purchase Order Number [number], 
dated [date]. We regret to inform you that said order cannot 
be filled at this time due to the following reason: Should you 
care to discuss this matter further, please feel free to contact 
me at the above number. We look forward to your future orders 
and are ready to assist you in any way possible. 

Yours very truly,