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Anti-spammers and others promote double opt-in as the only fail-safe way to build a permission-based e-mail marketing list. But one e-mail service provider recently found its messages blocked by a major Internet service provider because of the very confirmation process designed to prevent spam.

The problem: A so-called spambot—a program designed to collect e-mail addresses off the Internet—repeatedly registered an e-mail address on the e-mail service provider’s Web site. But the e-mail address was a spam trap and every time a confirmation e-mail was sent to the address, the ISP considered the message spam.

As a result, the ISP repeatedly blocked the e-mail service provider’s e-mail, said the company’s ISP relations executive, who asked that all names in the story be withheld.

“We were contacting them every other week to get delisted,” said the executive in an e-mail. “Finally, they invited us to join their feedback loop [a service some ISPs offer allowing e-mailers to monitor their spam complaints]. Lo and behold, we discover that the e-mails getting reported to them as spam were the double opt-in confirmation messages.

“We contacted the ISP, and told them it was some prankster that was just reporting one of our users as a spammer,” the executive continued. “But they said it was impossible, because that particular e-mail address was a spam trap.”

Anti-spammers and others have long held that double opt-in—or fully verified or closed-loop opt in—is the one foolproof way to ensure an e-mail list is permission based and keep the sender off blacklists.

Under double opt-in, a would-be e-mail subscriber must reply to a confirmation message to register for a list. As a result, double opt-in’s proponents say, there is no way a subscription can be forged and the method is the only foolproof way to protect against being labeled a spammer.

However, the spambot’s behavior in this story illustrates otherwise, said the ISP relations executive. “It means is that double opt-in isn’t as foolproof as anti-spammers say it is.”

The executive was at a loss to explain why a spambot would be programmed to register a spam trap on a double opt-in list, or how widespread the problem may be.

“I have my own conspiracy theories,” the executive said. “I tend to think that some spammer out there found out the address was a spam trap. I’m sure they communicate with one another.”

Someone may have decided to pollute the spam trap and make it unreliable by registering it repeatedly on non-spamming Web sites. “That’s my conspiracy theory, but I’m super paranoid,” the executive said. “It also could have been a complete coincidence.”

So what’s the lesson for everyone else? For one thing, a good ISP relations program is crucial, said the executive. “To me, it means really get in close contact with whoever is blocking you, and talk to human beings on the other end of the phone.”

The executive added that the spambot still hits the company’s site about once a quarter.

“All we can do is send a pre-emptive email to the ISP telling them: ‘It's back, please don't block us again,’” the executive said.

Meanwhile, the only foolproof way to guard against this spambot’s actions would be use a CAPTCHA—an acronym for completely automated public Turing test to tell computers and humans apart—requiring subscribers to type in the letters of a distorted image.

“Convincing people to switch to double opt-in is hard enough,” said the executive. “Double opt-in plus CAPTCHAS and you might as well ask them to do calculus.”