I work with email. I don't mean that I use email for work, I mean my work is email. So when I hear that someone has 650000 emails on a laptop, I'm intrigued. 650,000 emails, using industry standard averages and including attachments, comes to about 60 gigabytes of email. It's also a LOT of email. I download all of my email from my gmail account locally. I don't keep anything in gmail, every 15 minutes I pull from my account and when it hits my local machine it gets deleted from gmail. Why? I'm paranoid. My gmail account has been compromised more than once. I also keep my address book local, rather than in gmail. Again, I'm paranoid. That doesn't make it really any less likely that I'll be hacked, but does reduce a bit of the attack surface, and since my home machine is protected pretty well and I don't save passwords, the amount of data that they'll get is reduced to maybe this blog and my embarrassing fetish porn stash.
Back on topic, because I am a lazy, paranoid digital hoarder, I have ever email that I've sent or has been sent to me since 2008 stored on this computer. I have three email aliases and two different email providers. I have less than 15 gig of email. Including large attachments. Uncompressed. I don't have close to a half-million emails. My largest single archive, going since 2011, is 7 gig.
650,000 emails is a year's worth of email from a good-sized corporation, not a single-user's mailbox. Not even five-year's worth of email from a single user's mailbox.