All mortgages of real property situated within this state which are taxed by this article, and the debts and obligations which they secure, together with the paper writings evincing the same, shall be exempt from ad valorem and all other taxation by the state, counties, towns, cities, school districts and other local subdivisions of the state, except this article shall not affect in any manner the collection of any income tax payable in whole or in part from the interest received from such mortgage indebtedness. The exemption conferred by this exemption shall not be construed to impair or in any manner affect the purchaser of real estate which may be sold for nonpayment of taxes levied by any local authority.