It's not a new one.
Part of Virgin (The bit that used to be BlueYonder) use what used to be MCIS - Microsoft Commercial Internet Services - this was a service provider targetted package which provided SMTP, POP, IMAP, Authentication and NNTP in a single package.
I was involved in the first rollout of this pile of wotsit in the UK at Cable and Wireless back in 1998 - Microsoft ended up having to do 2 custom drops of SQL Server just to support the authentication needs of 50k customers.
Roll forward several years - Blueyonder are using MCIS for their email platform. MCIS doesn't have a very good reputation, but they blunder onwards with up to 20% mail loss as they go, but it sort of works.
Microsoft in time decide that MCIS isn't a good brand and rebrand it - it's now "Microsoft Hosted Exchange" which is effectively a tweaked version of Exchange itself scaled for ManyManyLots customers (BT Connect2Business had 1.1million users on it - don't know if they still have).
BT rolled Hosted Exchange 2003 a few years ago - by the sound of things Virgin are probably still on HE2000 and are forcing an upgrade.
Exim will scale to this size of user base, but it's only an SMTP server - BT Internet, prior to merging its mail services with Yahoo! was running Exim on its mail farm for 4 million users pushing something like 20 million mails a day. You still need a POP/IMAP server (BT used a customised version of Cyrus as I recall - it's been a while) and an authentication server, but LDAP servers are easy to get.
Thing is - if you want a *seriously* large messaging system you're really looking at what used to be Software.com's Openwave platform - it was designed to sit at the middle of a mobile phone network with tens of millions of mailboxes and punt messages whether they be SMS or SMTP.
Of course, the alternative is to craft your own - ask Yahoo!, Google et. al.
Biting the hand that feeds IT
