In every reference about coordinate ascent variational inference for the mean field family (Chapter 10 Of the book of C.Bishop Pattern recognition and machine learning, or the review article of Blei et al 2017, Variational inference, a review for statistician, or this discussion for instance) we find the main result which is:
$$ q^*(z_k) \propto\exp\left\lbrace \mathbb{E}_{-k} \left[ \log p(z_k|z_{-k},x) \right] \right\rbrace, $$ which, as $q^*(z_k)$ is a probability distribution function, only as a sense when $$ \int \exp\left\lbrace \mathbb{E}_{-k} \left[ \log p(z_k|z_{-k},x) \right] \right\rbrace\text{d}z_k < \infty. $$ However, to the best of my knowledge, this hypothesis is never discussed in those references. Does it always hold in the case of mean field variational inference, and if so, why? If not, when does it hold?