If you’re in the US, go to the UPS store near you. Have them notarize a statement that the check is in the envelope. Send it so it requires a signature on receipt. They can do all this in the store.

Alternatively… open a separate checking account. Use it solely for rent checks. Deposit your rent in it each month. Write a normal check and send it via normal post. If the management company is cashing them, you’ll have a clear singular record. If they try to pull something shady, you have two important things;

1. The money they are owed.
2. Proof that you acted in good faith to pay your rent.

That would likely be enough to stop eviction proceedings in their tracks.