In the past year the list has experienced a lot of delivery problems (mostly delivery problems to some large US-based ISPs). The problem in all cases is an over-aggressive mail filtering policies implemented by the ISP to prevent unsolicited commercial emails (aka "SPAM"). In each case, we try to work with the relevant ISP to make delivery of mail possible.
While some (very few) ISPs would take notice and let us through, most ignore our request for cooperation. Unfortunately, we can do very little when the ISP ignores us. In such cases, we continue to try to deliver mail, but if it keeps being rejected, the mail address will eventually be removed from the list automatically.
Some ISPs will make some of the legitimate email messages to their customers fail, rather than invest extra effort in improving their filtering technology. Apparently, they figure that dropping a few mails is a lot cheaper than caring properly for their customers.
Other ISPs are considering extorting payments for reliable delivery. The ATM list is a voluntary effort, and we do not think it is an acceptable policy for us to sponsor an ISP to deliver mail anyway - that is what their users are paying them for.
If you are stuck with an ISP that blocks some or all mail from the ATM list, you still have several open options.