ada.dhs.org is under attack by spammers. This site runs with a properly configured mail server that does not relay; all spam that seems to originate here are forgeries.
This site has been the victim of two kinds of attacks recently:
Recently, spammers have been using forged addresses with this computer's domain to send spam. As a result, lots of DSN notices had been “returned” to here, clobbering our spam filter. As a counter-measure, the mail server had been upgraded and reconfigured to check for spams in real time (system load permitting).
Evidence of identity theft can be seen in the list of items trapped in our spam filter (if the filter has not been recently cleaned), and in the list of emails sent to non-existent users (which passed through the spam filter).
A security hole was inadvertently created near the beginning of April due to careless programming and insufficient testing. As a result of the hole, spammers were able to hijack this computer to relay spam for about 1 day (ticket 119). The security hole was closed on 2003/04/08.
The owner of this site apologizes about the incident, for not acting on the security hole sooner when suspicious signs started appearing.
The “state of the art” of third-party relay attacks seems to have changed; this site was under a distributed third-party relay attack; spams were injected from far-flung sites from seemingly-unrelated places on different continents. It is possible that spammers are now using DDoS “techniques” to hide their true identities, i.e., hijacking vulnerable “drone” computers and then using the drones to in turn hijack vulnerable server computers. It is also possible that spammers are now internationally-organized, so that “spamming rings” would have “branches” of their illicit operations across the globe.
Such probes and attacks are no longer being logged, not because they have stopped, but because the server is no longer running the anti-spam checks in batch. Back when the probes were loggable, you could find misguided probes from well-meaning sites.
Update in June, 2003: A volunteer at my workplace recently found out that his computer had been the victim of a trojan attack, resulting in his computer being used to send spam. Thus, the first postulation (that spammers are now using DDoS techniques to take over “drone” computers, so as to cover up their tracks) seems to be the more likely explanation.