Is your server blocked from sending emails? This tool checks your server's IP address against the Spamhaus Zen DNSBL (DNS-based Blackhole List).
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This usually happens if your server has been compromised and used to send spam, or if you are on a 'noisy' shared hosting IP.
You must contact the blacklist provider (e.g., Spamhaus) and request removal after proving you have secured your server.