IP reputation

What it means

IP reputation can be checked before a message is even transferred. Microsoft's documentation says incoming mail first passes connection filtering, which checks the sender's reputation, and that most spam is rejected at that point. DNSBLs also answer per IP address.

Google's Postmaster Tools has an IP Reputation dashboard for addresses that send DKIM-authenticated mail to Gmail (SPF-authenticated when there is no DKIM), rated Bad, Low, Medium or High. A new IP has no history, and AWS notes that mail from such addresses may go to junk or be blocked until the IP is warmed up.

On a shared IP the reputation belongs to everyone sending through it; on a dedicated IP it reflects only your mail. M3AAWG notes that IP reputation is sensitive to changes in volume, which is one reason providers pool smaller senders on shared IPs.