It doesn't matter if a service contains unsubscribe links or not. ConstantContact has strict anti-spam policies and their SafeUnsubscribe(tm) links at the bottom of each email. That doesn't mean I don't regularly get spam mail via their system because yet another spammer harvested my contact information from my website. I found it easier to just block Constant Contact entirely as I don't think I've ever gotten a single requested email from them anyway using the info I posted here: http://johnhaller.com/jh/useful_stuff/block_constant_contact...

I don't think I've had the same issue with MailChimp, but if I do, I'll do the same with them.