Modularity criteria for multiple Eisenstein series

We give two criteria for a linear combination of multiple Eisenstein series of fixed weight to be a modular form, one in terms of its Fourier coefficients and one in terms of its $L$-function. The first criterion follows from an exact transformation formula under $S{L}_2(\mathbb{Z})$, which describes the modular defect of multiple Eisenstein series explicitly. The second criterion is a Hecke converse theorem for multiple Eisenstein series, which shows that the functional equation of the $L$-function alone already implies modularity.