A vertical packaging machine that includes a forming tube having an inner passage section and retaining means associated with the forming tube. The retaining means includes a retaining device configured to be displaced between a blocking position (P1) in which it at least partially blocks the inner passage section of the forming tube, and a release position (P2) in which it does not block said inner passage section of the forming tube. While in the blocking position (P1), the retaining device is arranged at a fixed specific height and is configured to prevent a product from passing through the inner passage section of the forming tube. Upon transitioning to the release position (P2), the retaining device is withdrawn from the path to be followed by the product.