Check if an IP address is listed on spam blacklists (DNSBL).
A DNSBL (DNS-based Blackhole List) is a list of IP addresses known to send spam or malicious traffic. Email servers check these lists before accepting email — if your IP is listed, your emails get rejected or sent to spam.
Compromised server — malware sending spam from your IP. Open relay — your email server allows unauthenticated sending. Shared hosting — someone else on the same IP sent spam. New IP — some lists flag IPs with no sending reputation.
Fix the underlying issue first (stop the spam, patch the vulnerability). Then visit the blacklist's website and request removal. Most lists have an automatic delisting form.
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