Store a large number of emails

Hi team,

I sometimes see email boxes that have between 10 and 40 thousand messages. That's a lot. I would like to solve this problem. Where is it best to store the mail that cannot be removed for some time. Add that the user must be able to find the email they are looking for. What would be the most optimal solution. I would add that there are shared caches that the entire group of users can access. Thanks in advance for help