Ask HN: Sorting 8k emails, any tools to make this easier?

My email archive currently sits around 8k emails. I'd like to clean it up a little bit, deleting irrelevant stuff as to reduce my (and my clients') exposure should it get leaked. At the same time, there is some stuff I'm legally required to keep (invoices for taxes, etc) so I can't just delete everything past a certain date.

Does anyone have any tips on how to tackle this, or know any tools/services that can help out?

Thanks!