They not only block domains, some also block TLDs [0], particularly the ones listed on Spamhaus' 'Top 10 Worst TLDs' [1].

Speaking of SpamAssassin, a blast from the past... https://lists.archive.carbon60.com/spamassassin/users/199808...

[0] https://en.wikipedia.org/wiki/Top-level_domain

[1] https://www.spamhaus.org/statistics/tlds/