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.