FAD-dependent sulfhydryl oxidase that catalyzes disulfide bond formation. The complete pathway for formation of disulfide bonds in intracellular virion membrane proteins sequentially involves oxidation of E10, A2.5 and G4 (By similarity). O2 + 2 R'C(R)SH = H2O2 + R'C(R)S-S(R)CR' Interacts with A2.5; this interaction involves formation of a transient disulfide-bonded intermediate, allowing disulfide bond transfer. Associated with crescent membranes, immature virions (IV) and mature virions (MV). Expressed in the late phase of the viral replicative cycle. Belongs to the poxviridae E10 family.