I am hoping that you got a receipt for the cashier's check from the issuing bank.  If you did I would copy the receipt and advise your landlord, in writing and by certified mail, how and when you deposited the check (in the night drop box made available to the tenants for deposit of rent checks) and advise the landlord - nicely - that you consider the rent for the month paid and that you will not be replacing the check if the office lost it until such time as the funds can be traced.  What may in fact happen is that he threatens to evict you and indeed starts a proceeding.  I do not think that he will be successful although it will be a hassle for you.  Good luck.