The email domain where the messages originate is from some sort of federated identity management system that was created in 2010 (here is a proposal deck [0] with technical details). Found this program simply by searching Google for the sending domain. Based on the guide for using this system [1] (see step 15) looks like this specific email address is the one that sends automated confirmation emails upon registration…
It's likely multiple different sources. I just noticed I got it as well on my personal email (which has custom domain) and I don't own any IP ranges.