Legal Question in Landlord & Tenant Law in Missouri

"Illegal" connotes a crime. The matter you described is a civil matter. It may be unllawful for a landlord to declare that a tenant is later paying their rent, when they are holding a check. The check is only good for six months., if they hold it that long, they may not be able to cash it, and you will have already paid that month's rent. That is assuming you got reciept when you gave them the check. A tenant in that situation (where the lease does not require a money order) shoudl possible agree to excahnge the check for amoney order (minus the cost of the money order and a $5 inconvenience fee) That will force the landlord to decide how bad they would rather have a money order.

Good luck