$e^+e^-\rightarrow e^+e^-$ is called Bhabha scattering. Let us only consider the tree level Feynman diagrams of this process. Apparantly, there are s-channel and t-channel diagrams as shown in the linked wiki page enter link description here. But there should also be a u-channel diagram obtained by modifying the "Scattering" diagram:
Sorry for bad hand drawing.