Maybe this one email would have been fine, but if it gets tripped, it’s not worth the headache.
If they specifically ask for documents that are not relevant or if their request is too broad so will produce a lot of irrelevant documents your company's lawyers will tell them no.
By the time someone is actually specifically giving you a list of things to turn over that includes your private email it will only be asking for things that are relevant. Most of your personal email will be excluded.
I'm not suggesting this is actually a problem, but that's how an argument could go.