About DNSBL checks
DNS blocklists (DNSBL / RBL) are real-time databases of IP addresses associated with spam, phishing or abuse. When a receiving mail server looks up the sender's IP in a DNSBL and gets a positive answer, it may reject or score the message. If your mail server's IP is listed, you'll need to request delisting from each blocklist operator after fixing the underlying issue. Note that error results are typically caused by public-resolver rate limits during the lookup, not actual listings, re-run the check if you see many errors.