By contrast, I once subscribed to another list that I assume must have been created by an inexperienced affiliate marketer who was clearly more interested in building lists than in providing quality content. I say this because that individual started blasting me with emails--up to THREE PER DAY!--and automatically added me to other lists I hadn't subscribed to without my permission. I immediately unsubscribed from all the lists and marked the emails as spam, and I'll never join another list for that company's products/services again ... which is a real shame for that company because, up to then, I had viewed them as a reputable source of information on the topic. Not anymore.
When I started creating my own lists, I received advice that I should send one email per day that contains valuable content free of charge, interspersed with items for purchase only occasionally (like maybe once every three or four emails). I've been doing this and it seems to be working. No one has marked any of my emails as spam, and I've had only a 2% unsubscribe rate.
What are your thoughts? How much is too much? Or does that vary depending on the industry and topic matter?