Why does IP reputation still matter for some providers?
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You might hear that IP reputation is old news, that domain reputation is all that matters now. That's mostly true for Gmail and Yahoo Mail. But "mostly" is doing a lot of work in that sentence.
Outlook (and Microsoft 365) still weighs IP reputation heavily through its SmartScreen filtering. Microsoft hasn't quietly dropped IP signals the way Google has. If your IP has a rough history, Outlook will notice.
Enterprise email gateways are the bigger blind spot most senders forget about. Barracuda, Proofpoint, and Mimecast sit in front of corporate inboxes and rely heavily on IP-based blocklist data. A lot of B2B email never even reaches the mailbox provider because a gateway blocked it first based on the sending IP.
Smaller ISPs and regional providers are another category where IP checks still run the show. They don't have Google-level machine learning systems doing nuanced domain analysis. IP reputation checks are proven, affordable, and good enough for their needs.
Then there are blocklists. Spamhaus and SORBS operate at the IP level. Getting listed there damages deliverability across every provider that queries those lists, regardless of how strong your domain reputation looks. It's a parallel track, not a replacement.
The practical takeaway: domain reputation is the main event for consumer inboxes, but IP reputation is still the gatekeeper for enterprise, B2B, and regional mail. If your audience is mostly corporate addresses, your IP health matters more than most guides will tell you.
Not sure where your IP stands right now? You can check it against major blocklists with our free blocklist checker.
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