I'm trying to read more on how they worked/work, but the articles are always a bit confusing.
I am never sure what kind of anonymity did they provide (or not), and what was the mechanism, and what were the "gotchas".
And, more importantly, if any are still active in 2020 and if not, why not.
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