In the early hours of this morning, a spammer managed to get the IP of the Gentoo list server on the NiX Spam RBL... simply by spamming the subscribe address :-(. This caused approximately 2000 deliveries of normal list mail to be rejected while the server was present on the RBL.( Log details )
Why did this happen? I do agree on the importance of spamtrap accounts, but they MUST check the content of their messages. A list confirmation message MUST NOT be considered as spam.
The original subscribe request came from what seems to be a compromised server in Secunderabad, India. So it wouldn't have been detected by RBL focused on modem/dialup addresses.
Short of raising the bar to subscribe (with a specific token that needs to be included, and then it's only a matter of time till spammers include it too), there isn't much we can do to block stuff like this at the list-server level. There is no way to detect than an address is a spamtrap. There cannot be by definition, as the spammers would avoid it themselves otherwise.