Spam filters learned that spammers pretty much use the same URLs and domains for their spam.
So they started to store the URLs that spammers use.
Then spammers learned that they can just keep making up new domains really fast.
Then spam filters started to block emails that contained "fresh" new domains with no history behind them.
Then spammers learned to take advantage of "redirecting" services like TinyURL (a great, free service you can use to shorten very complicated URLs into tiny ones).
Then spam filters learned to actually CLICK each and every single URL in every email they receive, in order to follow those redirect links and analyze the reputation of the final destination domain.
The problem with this is if you send a legit email campaign that has a one-click unsubscribe link, spam filters will click those links to check their reputation, which will accidentally unsubscribe some of your members.
It’s something we predicted could become a problem back in May 2007 (when we noticed this press release from Barracuda Networks). Now it’s November, and we’re starting to get confused users and their subscribers writing in to complain about being unsubscribed for no good reason. Analyzing server logs, it looks like Trend Micro is doing the automatic unsubscribing.
What’s a marketer to do? The one-click unsubscribe link is a best practice. To prevent list shrinkage, should we start introducing a two-step unsubscribe process? It would prevent these accidental unsubs, but also make it harder for people to get off our lists. I hope it doesn’t come down to that. Hopefully, spam filters will consider using "white lists" of URLs they can skip over.