I need to run some metrics on that and see. Next time I do a spam sweep I'll see if I can tabulate the most common relays responsible for the spam. One thing I can promise you -- it's not the paid ones. That's the ultimate solution to the spam issue imo.
Discussion
I deleted several relays that I used (currently I still only use free relays) and I received a lot of spam. Interestingly, I was thinking of doing a similar test today.
Agree paid relays are the solution but there’s probably some low-hanging fruit URL blocking that could be added to pihole-type lists that relays could access.