ESP unwittingly used to send spam

Late last week I heard from someone at AOL they were seeing strange traffic from a major ESP, that looked like the ESP was an open relay. This morning I received an email from AOL detailing what happened as relayed by the ESP.

IronPort Open Relay Vulnerability
Systems Affected
IronPort A60 running software version 2.5.4-005. According to IronPort, later devices and software versions using the same filtering mechanisms are vulnerable.
Overview
In recent weeks, one or more rogue spammers have been using misconfigured IronPort A60s as open relays to send unsolicited emails for AOL users via open relay. It is important for IronPort device administrators to review their configuration to shore up any vulnerability to this web server exploit.
Diagnosis
A seemingly minor configuration mistake made years ago internally has been exploited over the last several weeks to send out massive amounts of unsolicited email to AOL users. The spam mail originated from an outside zombie server, apparently infected with remote mailing viruses (such as BackDoor.Servu.76) according to the IT contact at IP 66.139.77.16. <ESP> has a filter specifically designed to deliver email over IP ranges set for AOL only. However, it was listed before a filter designed to log and discard bounced emails coming in through the Internet-facing of the IronPort appliance.
Impact
We have received 6,500 customer complaints so far through the AOL feedback loop. As the IronPort devices are black boxes, we are unable to determine how many unsolicited emails were delivered across them. It is difficult to ascertain whether or not the rogue spammer(s) knew only AOL addresses were delivered using this exploit. It is important to note that only AOL addresses were delivered in our specific case due to the order of the filters.
Solution
The solution was simple: move the filter designed to log and drop bounce messages coming in from the Internet to the top of the filter list so it will run first, as other filters may direct the IronPort device to deliver the emails through this vulnerability.
Authors: Jake Lanza, Baigh Auvigne, Daniel Fox

Congrats to the ESP for noticing this so quickly and being on the ball to stop this leak so quickly.
The compromise was first noticed when email coming back through the AOL FBL did not match any mail sent by the ESP. Initially, the ESP contacted AOL to report a problem with the FBL, but in working with AOL employees determined the email was coming from the ESP’s IP addresses.
This highlights the need to not just process FBL emails, but also monitor them and react when there are emails in a FBL that you do not recognize.
Ironport has responded here.