I did visit [www.whitelist.org](https://www.whitelist.org) and they do list
45.33.6.223 as level-3. I suppose that means that somebody else in the
45.33.6.0/24 block is spamming. That IP address is assigned by the
ISP (Linode) so there isn't much I can do to prevent this. I've had the
same IP address for over 15 years.
Whitelist.org offers to exclude 45.33.6.223 from the level-2 and level-3 lists
for $100 (90 CHF) for two years. But that seems like ransomware to me.
("Nice email domain you got there. It'd be a shame if somein' happened to it.")