Take a look at NetApp. My company (unfortunately) signed an NDA with
NetApp, but they've posted that Yahoo uses NetApp for their e-mail:
Rumor has it (no, I'm not violating my NDA) that Hotmail also uses NetApp.
-Plenty- of other large sites use 'em for e-mail. Just call a NetApp
sales person and and ask for the list. It's impressive. I do believe
it contains some carrier-class implementations.
We use Netapps at COLT for email.
And as I understand it, Demon Internet in the UK uses them also
and they have over 200,000 users taking SMTP and pop3 feeds.