[pferde]

Except that users are (or should be) trained NOT to click on any links in spurious e-mails they receive.

No, dear sites, kindly deal with a little bit of sign-up friction and stop offloading to others the fallout from people abusing your services.

[harryh]

It's not just the sites you are asking to deal with that friction, but also every single individual user who ever signs up for anything on the internet.

That's a lot of friction. Too much.