Some people didn't respond to my verification message. Does it work?
We've conducted extensive tests of Mailman's invitation verification system and found that when people sign up for a list on a Web site, and those people are immediately sent the verification message, about 95% of them complete the verification process. The other 5% are mostly people who entered the wrong address (in which case they shouldn't be added to your list anyway) or people who are using overly-restrictive spam filters that would also block the actual list messages (in which case there's little point in adding them to your list until they fix their filters).
In short, the verification system works very well.
However, the response rate can drop significantly in other situations. For example, in one case we saw, the response rate dropped to around 50% when people signed up for a band mailing list on a paper form in a nightclub, then were invited to join the list several weeks later. If this happens to you, it does not mean that there is anything wrong with the verification system. After all, these people are receiving the exact same verification messages that have a 95% success rate in other circumstances.
More likely, the lower response rate in such cases is a combination of improperly written addresses, people who write down the addresses of other people who they think would be interested but aren't, and people who simply change their minds when they're back at work in front of a computer that already has more mail than they can handle in their In Box.
In general, the longer the delay between the collection of the address and the sending of the invitation, the lower the response rate. Don't be surprised to get a poor response rate if you invite people to join weeks or months after you collect their address. Some of this will be due to more undeliverable messages (people often change e-mail addresses these days), but most of it is just human nature; the more time people have to think about something, the more chance they'll change their mind. The thing to remember is that these people are choosing not to subscribe: as we said, our tests demonstrate that almost all of these people are able to verify the subscription if they want to.
Customers sometimes tell us potential subscribers are "too busy" to click on the link in a verification message, or that the instructions are too difficult to follow, and ask us to make exceptions to our spam policy. We can't do that, though. Making exceptions would cause you far more trouble in the long run if any of these people later complained that they didn't want to be on your list in the first place and that you were spamming them: you would have no proof that the allegation wasn't true. And if we couldn't prove to a blacklist operator that the allegation was false, we'd have to shut down your list entirely to make sure our servers aren't blocked, and you quite definitely don't want that to happen.
Simply put, spam is such a problem today that you should be able to prove that anyone on your list asked to be on it. We firmly believe that it's not too much to ask that a person click on a link, or reply to a message, to indicate that they want to be on your mailing list. The system protects everyone involved: it protects recipients from getting unwanted messages, it protects you from false spam accusations, and it protects us from being listed in blacklists. Verification is an "inconvenience" in the sense that it's "inconvenient" to enter a PIN to withdraw cash from a bank: Yes, it's an extra step, but the extra step is a small price to pay to prevent problems.