Significance Mitochondria contain several hundreds of proteins. The mitochondrial content is regulated by the uptake and degradation of proteins. Stabilization of protein structure by disulfide bonds was proposed to drive protein accumulation in the intermembrane space of mitochondria. However, it remained unknown if structural alterations could lead to protein escape through the physiological barrier formed by the outer mitochondrial membrane. In this work, we present evidence for size-dependent retrograde movement of mitochondrial proteins to the cytosol. We identify the translocase of the outer mitochondrial membrane channel protein Tom40 as an exit route. Our results indicate that the retro-translocation serves as an important surveillance mechanism that regulates the abundance of intermembrane space proteins in response to changes in cellular physiology. The content of mitochondrial proteome is maintained through two highly dynamic processes, the influx of newly synthesized proteins from the cytosol and the protein degradation. Mitochondrial proteins are targeted to the intermembrane space by the mitochondrial intermembrane space assembly pathway that couples their import and oxidative folding. The folding trap was proposed to be a driving mechanism for the mitochondrial accumulation of these proteins. Whether the reverse movement of unfolded proteins to the cytosol occurs across the intact outer membrane is unknown. We found that reduced, conformationally destabilized proteins are released from mitochondria in a size-limited manner. We identified the general import pore protein Tom40 as an escape gate. We propose that the mitochondrial proteome is not only regulated by the import and degradation of proteins but also by their retro-translocation to the external cytosolic location. Thus, protein release is a mechanism that contributes to the mitochondrial proteome surveillance.