Can I use lists provided by third parties?
As far as spam policies are concerned, there is no such thing as an "opt-in" list obtained from a third party. Even in the extremely unlikely case that each recipient willingly agreed that the third party could share his or her address with people the recipient has never heard of, the recipients still have not given you permission to send them messages, as required by our Terms of Service. Almost every other bulk mail sending company has the same policy.
The reason you can't send bulk mail like this is simple: If you send mail to people who didn't ask to get mail from you, they'll report it to their ISP as spam. Their ISP may then block all mail from our servers, preventing our other customers from sending normal mail to that ISP.
By the way, if you’re used to offline marketing: We know this is very different from accepted offline mailing practices. There are good reasons for that, explained on a third-party Web page called Explaining the Spam Problem to Marketers.
Inviting people to join your list
If you're 100% sure that each recipient knowingly and voluntarily gave the third party permission to share their address with your organization, you can use the "invite" feature of the Mailman mailing list system we provide. This feature sends a message to all the addresses, inviting them to join your list. They can click on a link in the message to join.
This allows you to take advantage of the list in a responsible fashion: you can use the list of addresses as a starting point, but still verify that each person wants to receive mail from you.
(There will be some people who don't join, of course, but that's their choice: if they don't want and agree to receive mail from you, you shouldn't be sending it to them. Many of these recipients would complain if you just sent them messages, and you definitely don't want such people on your list anyway.)
Keep in mind that you should only invite people if you're sure they knowingly gave the third party permission to share their addresses. If the third party simply buried some fine print on a privacy page that few people are likely to read, saying something like "we may share your address with our marketing partners", that doesn't count as "knowingly giving permission", since most people probably don't know they agreed to it.
Ask yourself this question: "Will all the people on this list agree that they asked the third party to share the addresses with me?" The answer might be "yes" if, for example, you run a local animal shelter and the list came from a national animal welfare organization that clearly had a "share my address with local animal rescue groups" checkbox on their email signup page. In cases like that, you can use the addresses as a starting point, inviting them to join your own list.
But if the answer to that question is "no, some people will complain that they didn't ask for this", then the entire list is useless. It should not be used for Mailman's "invite" feature or anything else.