How do you prove you sent a security refund?

[Johann Jells]

A local landlord has run into a problem with an ex tenant trying to cause problems for him. Among other things, the tenant claims he never received a refund, while the landlord says he sent it but it was never deposited, probably because he had withheld for damages and rent due. Other than getting a receipt upon personal delivery of check, how do you protect yourself from this kind of nonsense?