可以寫得有禮貌d ma?

這句省去Therefore, it is most unlikely that you sent it.
因為你只能表達你一方沒有收到支票，而不應下定論是對方沒有寄。

這句亦要省去, as you say, it was indeed sent to us.這句亦是明示對方在說謊，亦是不合理的講法。

你可以用以下句代替。

Perhaps your mail and cheque was lost by the post office or being delivered to the wrong address by mistake and never return to you, if you don't have a return address in the envelop.

這句太直接了Therefore, you leave me no alternative but to insist
全句可用以下句子取代：
Therefore, as you may check that the cheque you claim sent to us is not debited from your account by this week, please issue another cheque to us so that this debt account can be settled.

貴公司的表達有點令人反感，凡事都應留一線，不要太主觀和定奪，例如寄失東西不一定要說是人家沒有寄，可能郵局漏派，寫錯地址，甚至是在打回頭中；另外事情不要要跟客戶決裂，代對方想多一步，要求對方重寄一張cheque並保證在他驗證前一張cheque沒有對現，給時間讓他也有下台階，亦表明貴公司是講情理的。你的原文好像是追上一代的欠債而不是一期的欠款。

好在你問吓，否則你肯定多了一個angry customer和一個unpleased boss.

Dear Mr Chan,

We acknowledge receipt of your letter dated 6th March but regret to inform you that we still have not received the cheque of 14th February as mentioned.

Please rest assured our accounting department has maintained a system for validation of incoming remittance in a manner that up-to-date payment record can be tracked in respective accounts.

We would appreciate if you could bring attention to this matter and, re-issue a cheque to us as deemed necessary without delay.

Yours faithfully,
XXX

kahei0507：你既明白須要寫得客氣些, 何不直接了當地當對方確有寄支票給你的公司, 但可能是寄失了的情况下向對方去信, 便不需失和氣及有提對方支付欠欵便可一舉兩得了, 但如此便要把原信內容修改如下：

Dear Mr. Chan,
With reference to your letter dated 6th. March the contents
of which I noted with respect. However, as we have not yet received the cheque that you sent to us on 4th. February, and that our accounts department has double checked to affirm that the aforesaid cheque had never been credited to our account, we thus assume the letter you sent to us on 4th. February must have gone astray.
Under the circumstanes, I would suggest you to check with
your bank and order stop payment of the aforesaid cheque so that another
cheque for the same amount can be drawn to us at your earliest convenience.

Yours faithfully,
XXXXXX