Hi,
They use lots of servers and IPs, in about one hour I got around 100 emails.
Once an IP address is blocked by Wordfence they use another server and IP.
Before it worked 100% and i've got no alerts, but after the last 2 or 3 Wordfence updates it seems to don't work properly.
It's not a complaint, just a report. The plugin works very well :)