Judging by their site, root eSolutions seem most likely to be the negligent enabler rather than the culprit
This is almost certainly true. Very likely the same thing with atvclub.msk.ru, as well (although I can't be arsed even to check what kind of a content they host). It might not even be a client of theirs, it's entirely possible - perhaps even likely - that there's either a single infected system, or part of an actual botnet behind this.
It would take a Herbie-level LOGIC FAIL to send kiddypr0n spam from something that is clearly registered to you, after all.
So, don't go sending letter bombs, that would be bad.
Feel free to annoy the heck out of their abuse-contacts, though.
If someone were to take akshual measures to find out who is behind this; those two would be next step. We can show the IP and the email address and date and time, but we don't know more. They know - or should know - who is behind that IP and address.
Too bad that I didn't save the URL of the site they were advertising. I haz a screenshot saved - should someone need proof of the post's contents, but since the URL was only a link, it's not visible. Although I really doubt that there would be a connection between that URL and either the owner of the IP range, or the owner of the domain the email was sent to.
