We present the first predictions for the widths of resonance states in a so-far unobserved exotic isotope beyond the proton dripline, 21Al. For this purpose we employ mirror symmetry between the widths of low-lying resonance states and the asymptotic normalization coefficients (ANC) of their mirror analogs. The latter are extracted from a recently measured peripheral reaction 20O(d,p)21O while the positions of resonances are estimated within a microscopic cluster model and a two-body potential model. We have found that 21Al should have two low-lying states decaying into the 20Mg+p channel and three states at higher energy, built on the 2+ core excited state and decaying into the 20Mg(2+)+p channel. The widths of all these states are much smaller than their energies, which makes experimental spectroscopic studies of this isotope beyond the proton dripline possible.