(17) Safelists (also known as mailers OR safelist mailers) have been in existence since the late 1990's, and I was doing them since 1999. (I just did SEO, EZINE advertising (AKA "Solo" ads) and FFA before that).
The "Safe email Lists", AKA Safelist, existed through the 2000's, the 2010's and now we are approaching 2020's. This safelist has ALL the latest knowledge (some not so well known knowledge also) in getting email ads delivered, DKIM+SPF headers, latest design, latest popular features and latest programming skills, plus all the decades of experience of owning and running them. So I call it 3rd generation.
Some people know them as a "Mailer", which is the very same as "Safelist", and it always has been. The terms have always been used for many years, as I had a safelist called "Webstars Rando Mailer" in 2005, another called HotSpotMailer in 2009 and some others in between there. There is no difference between the two names of it.
Safelists, when they first started was a safe email list using the scripting, servers, and admin maintained it. Some used credits, Some had limits. Some did not.
Mailer and safelist is the very same thing and there is propaganda taught to others that it's different.