Videotape yourself in the post office putting the check in the envelope, sealing it, handing it to the worker, paying for its certified delivery, and your receipt. That's the only way I can see that you can prove the rent was mailed.

Maybe there's a service where you can transfer the money and someone can hand deliver a check to the landlord. Then that service could vouch that it was delivered if the landlord says it wasn't. Like a service that delivers court papers.