Energy levels of carbon-like iron Fe XXI, with configurations 2s²2p², 2s2p³ and 2p⁴ are calculated from the multiconfiguration Dirac-Fock method. The relativistic electron-electron interactions represented by the Breit operator are calculated with wavefunctions of definite angular symmetry instead of averaging over configurations. The authors find that the Breit interaction substantially improves the agreement between theory and experiment on the fine structures of the triplet states, while the Lamb shift corrections are very important in attaining accurate transition energies between configurations. The authors results on energy levels agree with experiment to within 1% for all of the eighteen levels compared, whereas corresponding non-relativistic results are off by 10-30%.