Genuinely: why? All that is related to the business email is already accessible, it's just been forwarded elsewhere. The info is already known. What's to discover?

To discover where else you then subsequently forwarded it.

I'm not suggesting this is actually a problem, but that's how an argument could go.

It indicates that you used your this other email for work purposes. The point is that they don't know what's in there, and they want to see if you discussed stuff relevant to the case. The judge will deny such a request as a fishing expedition if there is no basis for believing that you used your personal email for work purposes. But if it is discovered that you started sending work emails to that address...

Assuming we are talking about discovery in a civil lawsuit involving your employer the party opposing your employer can ask for all documents you have that are relevant to the lawsuit. It is then up to you to turn over those documents.

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.