Email Piggybacking other sites

Here's an idea for those that have successful email campaigns: We each give our subscribers a link at the bottom of our e-newsletter to subscribe to (or get a sample of) the other sites' e-newsletters. There would have to be some verification of the size of the lists, and we don't want competitors to get any of the links. Other than that, I don't see any downsides to this. Obviously, nobody is giving up their lists, but just the opportunity for our people to subscribe to theirs and vice versa could be a win/win/win... situation. Any feedback?