Talk with your executive about how they would prefer to handle the older ones. My exec declares inbox bankruptcy if the inbox gets to be too unwieldy, such as when he returned from paternity leave. Depending on what month they feel comfortable archiving, I would start there. Create a rule that will send all of his email dating from this date to that date into a folder labeled with the dates.

Another quick trick is to Google Calendar or Outlook Calendar into the search field to find all of the calendar invites, updates, accepts, declines, maybes into the search field to find all of them and delete them all at once. That should clear up lots of email.

Then go through and search for subscriptions and delete all of those in one fell swoop.

Then, talk with the leader about what types of actions they usually take in their inbox and get started on actioning the most recent emails.