Unenforceable contracts are those, which are very much valid contracts, but simply cannot be enforced in a court of law because of the absence of some essential legal requirements or evidential features. Stamp act requires that insurance policies are to be appropriately stamped. If therefore, a policy is not stamped, then even though the contract will be valid but such a policy cannot be produced in a court of law as evidence to the contract.
This could happen because the terms of the contract are ambiguous, if one party has a voidable contract or if the Statute of Limitations has expired. Contracts may be unenforceable because of their subject matter, because one party to the agreement unfairly took advantage of the other party, or because there is not enough proof of the agreement. It is a valid contract that cannot be fully enforced due to some technical defect.
Example: Bill bought a property from Harry through a written contract for sale. Seven years after the purchase Harry wanted to claim that the contract was unenforceable. The statute of limitations for written contracts in Oregon is six years and Harry would not be able to challenge the contract.