Vendor lineage
IronPort Systems
Worked out that the useful question about an email was not what it contained but who sent it.
IronPort was founded in 2000 by Scott Weiss and Scott Banister to build email security appliances at a moment when spam was moving from an annoyance to an operational cost. Its answer was not a better content filter but a different question entirely.
Content filtering asks whether a message looks like spam. That is an arms race, because the sender can rewrite the message. IronPort's SenderBase asked instead who was sending it, and how that sender had behaved everywhere else - volume, complaint rates, how long the address had been sending mail at all. A sender with no history and sudden volume is suspicious regardless of what the message says.
That reputation data was aggregated across every customer, which made the network itself the product: the more organisations used it, the better the signal. It is the same structural argument that cloud-delivered security has made ever since, and IronPort was making it about email in 2002.
The appliances ran a purpose-built mail transfer agent designed to hold enormous numbers of simultaneous connections, because a mail gateway under a spam flood fails on connection handling long before it fails on filtering.
Cisco announced the acquisition in January 2007 for around $830M. The technology became Cisco's Email Security Appliance line, and the reputation approach outlived the brand by decades.