The search for the existence of solar axions has been and continues to be brought forwards using so-called helioscopes. In a helioscope, solar axions can travel in an evacuated volume in presence of a strong magnetic field. Along this path, axions have the possibility to convert to photons. Detecting such photons, expected to be in the keV range, will be the indication of the existence of solar axions. X-ray detectors with low background, high efficiency and high energy resolution are therefore an important component of such experiments. Metallic Magnetic Calorimeters (MMCs) are detectors operated at mK temperature which offer an energy resolution better than 10 eV full width at half maximum (FWHM) and a detection efficiency close to 100% over the energy range of interest for solar axion, including a spectral line around 14 keV generated in axion-nucleon interaction in the Sun. In addition, a very low energy threshold can be achieved, limited by the necessary window against infrared radiation. These detectors are therefore one of the technologies considered for the fourth generation helioscope IAXO. The present status of MMC optimization for IAXO will be discussed along with the importance for both the axion discovery phase and even more for the study of axion properties after discovery, thanks to the unique performance of these detectors.