In the past decade, the remarkable improvement in the performance of millimeter-VLBI arrays has led to important progresses in the investigation of jet formation in active galaxies, recently culminated in the imaging of M87 on event horizon scales. While high-resolution studies have been performed so far in a few bright radio galaxies besides M87, the boost in sensitivity due to the use of wide bandwidth receivers and to the inclusion of large telescopes is now opening a new window in the exploration of AGN cores, enabling us to study fainter objects for the first time. Probing the jet formation region in a larger sample of sources is fundamental for testing theoretical models describing the launching, acceleration and collimation of the relativistic plasma in different luminosity classes. In this talk, I will present results from a millimeter-VLBI studies of previously unexplored sources, which could be imaged with a resolution down to ∼100 Schwarzschild radii. This enabled us to probe in detail the jet collimation region, determine its extent, and infer the physical conditions of the plasma in the vicinity of the supermassive black hole.