Unknown error: permanent error (550): 5.7.1 Rejected by spam filter

I’m getting tons of messages with this “Rejected by spam filter” error. Is this my mail server’s spam filter (I use ProtonMail) or is this Cloudflare? I was under the impression that Cloudflare just does a straight passthrough without inspecting anything. Prior to moving my MX to Cloudflare, the MX was pointed to ProtonMail and I was receiving these same messages (in the spam folder). Yes, they 100% are spam, but I just want to understand what Cloudflare is doing upstream.

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Seems like CF is using UCEPROTECTL3 blacklist and this is a mess. Only owners of the ASN can request a removal or you pay an annual subscription. Even parts of the ip range from CF is marked as higth risk.

host route3.mx.cloudflare.net[162.159.205.23] said: 554 XX.XX.XX.XX found on one or more DNSBLs (uceprotectip3). Refer to https://developers.cloudflare.com/email-routing/postmaster/#spam-and-abusive-traffic for more information.(in reply to RCPT TO command)