No liability exists if the party who interferes with the performance of a contract between two others did not know of the existence thereof.1 If, however, such person, while he does not know of the existence of the contract, knows the facts which put him upon inquiry, and if due inquiry would have disclosed the existence of the contract, such person is liable for interference therewith,2 even if he omitted to make such inquiry.3 Actual notice or knowledge of the existence of the contract is unnecessary.4