Spamhaus currently includes 225,454 U.S. based Internet addresses on its Composite Block List. Of those, nearly one-quarter — almost 56,000 — are assigned to Verizon.net.
Part of this could be because Port 25 is still open and allowed for sending email on Verizon’s network. Almost all other ISPs no longer allow this and have stopped allowing it long ago. In fact, you may remember when you set up your email that you used port 587. This port requires you to authenticate with the server.
One of the first things that will happen with this shift is that Verizon will be able to quickly identify zombies (because they will still be attempting to send email on port 25) and shut them down.