That's what Yahoo uses right?
Sent from my Windows Phone
I'd imagine that some technology from Zimbra made it into Yahoo Mail, and
vice versa. For a while, Yahoo owned Zimbra, but it's a VMWare product right
now.
I know that Zimbra will scale well. How exactly you scale Zimbra would
depend in part on whether you choose to go with the FOSS version, or with
one of the commercial packages.