If you trust your landlord, I would write a second check and ask them to tear up the first one if it should ever arrive or hold it for next month's rent. But if you're really worried about them or somebody else cashing it, put a stop order on it. Yes, the fee might be stiff, but I assume it's less than the amount of the check.

How far away was the mail traveling? I would think something mailed on the 30th would have already gotten there by now, but I suppose it's possible that it simply got hung up. Any chance that you addressed it incorrectly?