The best free tool is Google Postmaster Tools (postmaster.google.com) — it shows your domain’s reputation rating as seen by Gmail: High, Medium, Low, or Bad. For Microsoft/Outlook, use SNDS (sendersupport.olc.protection.outlook.com). For general blacklist status, use a blacklist checker like DomainScan’s — being listed on Spamhaus or Barracuda is a proxy for serious reputation damage. There is no single universal reputation score; each provider maintains its own.
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