Mail that is over a year old is probably irrelevant by now, but may still need attention in some form at a later time once the email is under control. Therefore, I would start with current email and work backwards as mail received today is relevant.
Sub folders are a start, but unless you have a filing system that is understood, that too may get out of control.
One place that your executive can start is to delete email that is junk or personal....this will trim down the task a bit. If you notice any mailing list emails, discontinue this practice....unsubscribe from recurring emails if it is not necessary to the job at hand and will minimize the task of managing the email load.
As email is responded to, you can file the original along with the sent response immediately.
As more email comes in, you can apply filters to place the email into folders as they come in....this will help you in organizing....but be careful with filters as they can filter unwanted things too.....make sure that your rules work.
Just a few suggestions
If you are satisfied, please check yes above to accept this answer. Thank you very much.
As always bonuses are always welcomed and highly appreciated.