20 ways to spot the work of paper mills: https://retractionwatch.com/2021/02/09/20-ways-to-spot-the-work-of-paper-mills/
However one, using a non-institutional email address, is not "a bad global habit", but deliberate. I have no thought of moving but do not want my entire professional life tied by email to my employer. My UCI address keeps student emails private but I tend to use gmail for off-campus concerns such as publishers. And not all scholars have institutions who can provide emails. If they refuse my email, I refuse to publish with them.
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