too harsh for a letter

Could you take a look at this letter composed by one of my students, please?
He is a very clever, as well as funny, student. But this time he overdid it, I mean, his letter is too harsh. My corrections are in brackets.

Dear sirs
It’s no need to point out the fact that you made (I think – have made) several serious mistakes during this period. As you understand, we are dissatisfied with it (I think – the current situation). Our production line works on a “just in time” principles. It means any delays like in a situation with our last order 322345 from (I think – of 12.10.2010) 12.10.2010 makes serious damage to the production process and cost much more than your “excuse us”! We don’t want to wait while your service stop being so poor and employees from your sales departments start working properly! In fact we are not dissatisfied, but mad with your performance, and especially with your plentiful excuse letters, with no information, which looks like a spam (I think - some spam) created by computer. For us it’s not even a question whether you can guarantee delivery dates in future or will continue to just write non-informative excuse letters, which include no decisions, excuses only, so this letter is to inform you that we are now looking for another supplier!

Yours sincerely

Thank you in advance

I agree that the entire tone of the letter needs to be changed - from the very outset.
The letter would also benefit from a clear introduction - there is no indication with regard to what the letter is in reference to.
Your corrections only highlight some minor points. This letter would benefit from a complete re-write.