All notices required by this Code shall be written or printed notices and shall be served personally upon the persons entitled to notice, or their agents, or by sending the notice by mail to the person so entitled to notice, or to his or her agent, if the residence or business address of the person is known, or by reasonable effort can be ascertained. If the address of a person can not be ascertained, then the notice shall be sent to the address of the person who last paid the taxes upon the property in question. A failure to give any notice required by this Code shall not impair or affect the validity of any assessment as finally made.