900,000 ISP customers blacklisted

Although the article does not mention this, the activity that is blocked for those addresses are attempts to connect via Simple Mail Transfer Protocol - i.e. only those running personal email servers or connecting to third party email servers using SPEWS would be affected.

SPEWS has given the whole of Blueyonder a level 1 listing (SPEWS S3198 covering 850,000 IP addresses) when typically a level 2 is used to cover IP addresses not actually used for spamming. On the other hand, Senderbase report that 16808 addresses in BlueYonder's space have been sending emails, with many exceeding 10,000/day (magnitude 4).

Given the spread of spam-sending zombies, ISPs really should be blocking port 25 by default while giving individual users the ability to set up exceptions where needed (e.g. a webpage where they could list the address of their company email server or any other systems they need to connect to via port 25).