I would think that if you can prove the theft, and if you can enlist the client’s bank (via a request from the client, because it’s in the interests of all parties to resolve the issue) to validate the criminal conversion, then the badly acting bank should make you whole. This supposes that someone outside of your circle of business associates and/or family who had legal access to the mail and to the check, and who may have had authority to act on it: to cash or deposit it on your behalf, performed a criminal act. (If it’s one of your associates embezzling from you, then that would be a different story, I think, unless a bank official was in on a conspiracy.) Banks are required to know check endorsers and to exercise reasonable care in cashing and honoring checks – particularly when the people presenting them are NOT the people named as recipients. If a bank honored a check made out “to you” for someone else, then they acted badly, it would seem, and need to overhaul their processes, at the very least.