How does IP choice affect sender reputation?
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Mailbox providers track reputation at two levels: the IP and the domain. Your IP choice determines whether you own that IP-level reputation independently or share it with other senders.
On a shared IP, you inherit the pool's existing history. That's helpful when you're starting out (you get a track record on day one), but it also means you're exposed to what other senders in the pool do. If someone else triggers complaint spikes or hits spam traps, your mail can get caught in the fallout, even with clean practices.
On a dedicated IP, you start from zero. No inherited reputation, positive or negative. Everything you send from that point builds the IP's standing from scratch. That's why warmup matters: you need to establish a sending history before mailbox providers trust you at volume.
Domain reputation has become at least as important as IP reputation. Your sending domain carries signals across IP changes. If you move from a shared pool to a dedicated IP, your domain's history travels with you. Strong domain reputation can partially compensate for IP issues. Weak domain reputation is hard to overcome even with a clean IP.
The practical implication: don't think of your IP as your reputation. Think of your domain as your reputation, with the IP as one factor that influences it. Use Google Postmaster Tools to see domain reputation separately from IP, which gives you a much clearer picture of where you actually stand.
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