Still active? Yes, the Type I (cypherpunk) and Type II (mixmaster) remailer networks are still active as of September 2020. http://echolot.theremailer.net/mlist.txt
You send an encrypted email #1 to relay A. When A decrypts this, it finds the message "Dear A, here is another encrypted message #2 which I want you to forward to relay B." Relay B decrypts anonymous message #2 to get message #3, which it forwards to relay C. Eventually the last relay is told to send the result to a destination email address D, which is the actual recipient. Relay A never knows that the message was going to D, and relay C never knows that the message came from you.
That seems to have the same problem as Tor though, right. If someone controls most of the chain - as is easy with 7 online remailers, he sees what came from who.
edit: and anyway, that page doesn't seem to show actual IPs of the remailers, google doesn't help that much
You send an encrypted email #1 to relay A. When A decrypts this, it finds the message "Dear A, here is another encrypted message #2 which I want you to forward to relay B." Relay B decrypts anonymous message #2 to get message #3, which it forwards to relay C. Eventually the last relay is told to send the result to a destination email address D, which is the actual recipient. Relay A never knows that the message was going to D, and relay C never knows that the message came from you.