The short answer is that there is no obvious way to distinguish between spam and a mailing list. The only difference between unsolicited advertising and opt-in advertising is the subscription list; the email system has no way of accessing such a list, and even if it did, no way of verifying its accuracy.
The way spam is distinguished in practice is through a number of factors including: the content of the message; illegitimate message headers; the number of recipients; and even the response of the recipients (for example, gmail users can mark messages as spam).
When sending messages to a list, one cannot just assume that opt-in is a blanket and in-perpetuity licence for advertising. A successful newsletter will contain useful content (not just advertising), will be infrequent, will have a valid reply address, will have an easy one-click method of opting out, will not contain tracking links, and so on.
The ideal spam filter would reject any email we don't want to receive, regardless of whether it is "opt-in" or not. So when sending mass emails, the key is to sending an email that you know that all the recipients will want to receive; if in doubt, don't send it, or weed the list.
The companies that send advertising are only successful when they respect their customers and are conservative in their email practice. There are no conspiracies or special agreements.
Last edited by neonsignal; 01-29-2012 at 08:00 PM.