How do I defend against a condo association that is suing me to collect payments that they lost

You will simply need to keep looking until you find a lawyer to represent you. Bear in mind that in most instances you will need to pay for the legal fees "out of pocket" as few such cases are taken on a contingent fee basis for variety of reasons. That said, delivery reciepts are only useful if there is evidence of the contents of the envelope. You might consider making a cell phone video of the check, you placing it in the envelope and then sealing it with the certified mail card, etc. Otherwise, resolving this shoudl be as simple as putting a "stop payment" on the check and reissuing the check if they claim they never got it. It should be easy to determine if the check was stolen and cashed by someone else just by checking your bank account.