18.104.22.168/16 belongs to Nobis Technology.
We've had spam from at least 20 different IPs in this range over the course of a year. We've had them blocked for a month, then three months or so. I think it's time to block them for a year.
Their website says they are involved with "very lucrative Internet services". I wonder if spamming is one of these lucrative services.
Google search for "nobis technology spam" is quite revealing; other people have trouble with them too.