Every so often I will find myself on a mailing list that I know I didn't sign up for. This always gets me very cautious and I treat the junk mail as hostile spam - don't click, just delete and ignore. However, after regularly receiving mail from the same address (usually newsletters or training advertisements), I'll usually investigate to determine if I can safely unsubscribe rather than have to put up with junk in my inbox.
When I examine the Unsubscribe links in the mail, they'll often go to well-known email marketing sites like Constant Contact or Mail Chimp. I take this as an indication that it's generally safe to follow that link, and I will usually unsubscribe that way, especially if there are other indications that the source of the junk is legitimate. This way I only ever go to the website of the marketing company, not the originator.
Is it reasonable to assume that spam coming through legitimate email marketing companies can be treated as safe, or at least safe enough to unsubscribe? By unsubscribing, do spammers learn that my email address is active? If it is safe, is there a good way to determine which marketing companies are trustworthy enough that I can feel safe unsubscribing?