(updated Webmail.us success story. We've changed our name to Mailtrust, and the stats below have been updated)

Mailtrust specializes in hosting email services for businesses. We host two products: Noteworthy (a Linux based POP/IMAP/Webmail/Calendaring service), and Microsoft Exchange. All solutions are protected by inbound gateway SMTP servers that run ClamAV on ~45 dual-Opteron servers with RHEL 5, protecting 700,000 email accounts, and ~30 million messages per day (as of January 2008). New virus definitions are pushed to all servers in real-time as updates are released by ClamAV.