It's not possible. The current AI detectors are extremely unreliable, whatever people selling them might tell you. You'd probably get fewer false positives by filtering the people who send mail that you think is AI generated.
There is no need to use ChatGPT for that. I suppose running the emails through LanguageTool (or, if you don't care about privacy, Grammarly) and counting the suggestions to fix "too wordy sentences" would be a good approximation.