Ask HN: Cross Platform Email Archiving

I'm on a mac, and have several email accounts, like most people. I'm using IMAP for everything, but some accounts are over 15 years old, and my email app (Mail.app) is getting slow. I can move email apps, but I also feel like it's a good thing to clean up / archive email or files. What's a good way to archive email? Preferrably crossplatform? - I could export all emails to .eml files in a disk image. This would allow spotlight to index it, which might come in handy in the future. - I could use mbox format if that's supported, but I'd also need a viewer. - On windows, people use .pst files, which would be great, but then again, I'm on a mac, and don't like outlook.