If the mortgagor or his special agent or attorney fails to set up a defense as provided in Code Section 44-14-184, it shall not be competent for any third person to interpose a defense and neither will the court itself, of its own motion, do so. However, one who purchases mortgaged property prior to the commencement of statutory proceedings to foreclose and who is not a party to the proceedings is not bound by the judgment of foreclosure and may, when the mortgage fi. fa. is levied, go behind the judgment and set up the defense that the mortgage could not be legally enforced against him whenever this is necessary and proper to establish the claim of title upon which he relies.