One example is an unsubscribe link. Legally, it would be no bueno to have it behind any sort of login.
Another is just counting if a link from an email was clicked. I want friction to be as little as possible. That's done by having some sort of redirect, but you have to use a JavaScript initiated post to weed add false positives. That's already ridiculous, but because of automated link prefetchers, you still need to disable that and show a f'n button.
And then I have to answer to clients that want to know why their clickthrough stats are down precipitously and I don't honestly have the wherewithall to explain the inner workers of every filter that snoops their email before they read it.
