It may not be the landlord mistake.

Like how do you keep a record that teh landlord receive the money.

It is your liability to keep a record and prove it is paid.

As a landlord and property manager, I know how complex to manage the propertys. If one day I receive a money order that does not have memo to say which address should be linked. How can I know who paid for?. A big manager may receive more than 100 checks every month. It is very hard to manager. So it is your mistake does not keep a record.