Google mail offers me 6.7 GB of storage and counting. Your IT department probably sends nagging emails once you hit 200 MB. Why?
Your corporate mail runs on Exchange. Microsoft makes money by selling licenses for Exchange to run on a single machine. Gmail runs on the Google operating system, so the work to deal with your mail is spread across an arbitrary network of machines and storage. They’re already dealing with insane storage and performance requirements to run search, and they can largely reuse the same technology for mail.
With Exchange, dealing with scaling is left to your local IT administrator. They buy a single fast machine with speedy RAID drives, and any upgrade to capacity or speed means moving everything over to new hardware. This makes it vital to avoid filling up the disk, and the only way to guarantee that is to put a hard limit on the size any user’s mailbox can reach. It’s not only disk capacity that’s constrained, the high cost of CPU cycles in this setup means that new algorithms to do interesting things are hard to justify.
This all sounds like a textbook case for software-as-a-service. So why aren’t all companies running on Gmail? There’s things missing from the Google services that Microsoft offers, but I think the real blocker is the business importance of email. When mail stops working, modern businesses are crippled. For something that vital, CEOs want to know there’s someone they can fire if it breaks. As an outsourced service, and with their consumer focus, it’s hard to feel confident that someone will be sufficiently motivated to solve your problem right now.