Both contain a list of people willing to be emailed other peoples offers. There isn't fundamentally that big of a difference that I can think off. Viral Mailers...the new Safelist
In my opinion, and through my experiences... Safelists are garbage. Its mostly a bunch of spam, and no one really opens much mail. The only reason people do is to get credits to blast out their offer. Just circular logic, and a waste of time. If you are going to market via email... Just go with a good solo ad provider. I know a few excellent sources, but that's just me. To the top, Zach Andary
