Spamcop
Like others on here (I suspect), I've used Spamcop to filter some of my e-mail and block the junk. It's done a superb job over the years, filtering out several hundred thousand spams over that time (it keeps count!). About one in ten "blocked" mails would be genuine - so I'd tell it to release those and "whitelist" the sender. The rest get reported to the source ISP (when that can be identified).
While I was in hospital recently, the webmail provider used by Spamcop closed down. The new owner of Spamcop has switched off the "mail filter" connection rather than implementing a new one. If Spamcop incorrectly blocks mail, that mail is lost. There's no way to see what it was, or to do anything with it.
The claim is that everyone received several e-mails warning them that this is going to happen. The reality is that I didn't (I suspect others didn't, either).
Spamcop will still do its traditional "report spam to the source ISP", which is of some limited value. The great bulk of what I report goes to the same half dozen ISPs (well-known spamsites in China, Brazil, Argentina, Russia, and the USA). About ten times in five years, I've had an acknowledgement from an ISP and been told that the offending account has been closed, so it's not quite been a complete waste of time.
I won't be renewing my sub next year!