There are an increasing number of questions and complaints about the quality of the open access publisher MDPI. Notifications have been received of submission processes lasting only a few days, which is too short to properly conduct peer review. In addition, many MDPI journals pursue an aggressive author recruitment strategy, often offering considerable financial incentives to publish in their exponentially growing number of special issues. And referees find that articles they reject are published anyway. Be careful when choosing a journal from this publisher if you want to publish Open Access!
The Directory of Open Access Journals (DOAJ) provides some guidance. A number of MDPI's journals (133) have an OA seal visible in this database. But a larger number (316) do not. There are more websites and tools available to help you identify predatory journals. Check this guide for insights and practical advice on how to avoid questionable and predatory publishers.