Your legal counsel will be the best source of answers to your questions and a lot will depend on state law and what documentation you relied on to conclude that John Smith had the authority to use the trade name XYZ Company. Also, was the check payable to XYZ Company, Inc. or just XYZ Company? Wass the bank presented with a properly filed assumed name affidavit or similarly required document to use a trade name in your bank's jurisdiction.
Laws vary by jurisdiction so we will not be able to answer your question with anything but speculation.
_________________________
Sola Gratia, Sola Fides, Sola Scriptura, Solus Christus, Soli Deo Gloria!
www.tcaregs.com