Quantum communications promises reliable transmission of quantum information, efficient distribution of entanglement and generation of completely secure keys. For all these tasks, we need to determine the optimal point-to-point rates that are achievable by two remote parties at the ends of a quantum channel, without restrictions on their local operations and classical communication, which can be unlimited and two-way. These two-way assisted capacities represent the ultimate rates that are reachable without quantum repeaters. By constructing an upperbound based on the relative entropy of entanglement and devising a dimension-independent technique dubbed "teleportation stretching", we establish these capacities for many fundamental channels, namely bosonic lossy channels, quantum-limited amplifiers, dephasing and erasure channels in arbitrary dimension. In particular, we determine the fundamental rate-loss trade-off affecting any protocol of quantum key distribution. Our findings set the ultimate limits of point-to-point quantum communications and provide the most precise and general benchmarks for quantum repeaters.