On a side note, it is illegal for anyone other than a postal worker to put something in your mailbox.

The tenant normally gets a receipt from you, doesn't she?

Does the mailbox in question show any signs of forced entry? (In other words, does it at least hint to some truth in the tale of theft?) Or does it look exactly as it would have looked if the envelope was never inserted into it (In other words, your word against your tenant's word)

One suggestion: If they want to pay you in cash have them make a deposit into your savings account. Its troubling giving out your account number, but perhaps you can make some kind of arrangements at the bank.

Thanks to all. Some answers to your questions: I have not been writing receipts for rent, although perhaps I should start. There is no lock on the mailbox, a thief could just walk up and take whatever is in there without difficulty. And I live next door in the same duplex, so insisting that cash payments be made only in person seems like the easiest solution for the future (but please let me know if I am missing anything).

Relevant info: Has this tenant ever paid with a check before, or has it always been cash? And has she ever put cash in the mailbox before?

Most places that I've paid rent don't take cash as payment at all. If your tenant doesn't have a bank account, then they can pay with a cashier's check or money order. Cash is too risky and untraceable.

@Snowman: It says that payments will be made by check or "certified funds". (I don't know what "certified funds" are; I used a boilerplate residential lease for my state which I found on the internet.) That said, I did accept cash in payment of rent from the same tenant previously.

My opinion might be different if I knew your tenant, but this sounds like a con to me. The giveaway is that the tenant forced you to essentially call her a liar in order to contest her story by claiming she told you she was going to do this. This is a gaslighting technique.

What does your lease actually say, contractually?

Anonymous: I'm tempted to agree with @DavidSchwartz here. Did you happen to ask your tenant what your (precise) reply was when she told you she was going to leave it in your mailbox? If so, did she remember exactly what your reply was, or did she just vaguely say that you were okay with it? Unless she actually recall the exact words or sentiment you showed, I wouldn't believe that. Also, note that a lack of a negative response does not imply an affirmative response.

It is also illegal for anyone but you or a postal worker to remove anything from your mailbox. Report the matter to the police, a crime has been committed. I do not understand why she has not reported the matter. It makes me not believe her story at all. I have rented all my life, at over 30 locations, no one is allowed to pay with cash. Everyone over here knows this. Of course, other countries may have different laws. I suggest you put it into your rental agreement that only check or money orders are accepted from now on.

Eat it up for now , and notice the police (inform first you tennant that you will fill a report for stolen goods, ask them when would they be available to testify that the cash existed) be polite, and inform them than to avoid this payments should be made by person, nothing personal. She might (or not) feel guilty if she's lied, but whether she does, you'll get the rest of your rent from that point. Pressing option 2 might be killer for a student if she's saying the truth, ~1500$ don't fall down from the sky.

First, ask your postal worker if they saw the envelope. They may have a policy that describes what they do if they find a non-postal-delivery item in the mailbox. I.e. the postal worker may have removed the envelope, and it may be sitting in the post office waiting for you to claim it. Second, if it's not in the dead letter office, you're in a situation where you only have word of the tenant. How you proceed based on that is up to you. Legally, they have not paid you and they owe you rent. Third, modify your lease agreement to require a check or electronic payment.

@MatthewWhited That seems border line spam-y, but yes, a cheap, electronic money transfer service is something I do wish more people would use so I could stop using checks. Ugh, paper.

It's not meant to be... I find it to be really handy. I like the fact I don't have to give out my bank info to third parties like I would with ACH transfers.

@bobbym why would she? If she does not know the landlord did not receive the rent (and how would she know that, other than she did not put it there), she would be ignorant that a crime had been committed. If she had not paid the rent, the 5th Amendment protects her.