Think of sender reputation like a credit score for email. Mailbox providers track your sending behavior over time and use that history to decide how to handle your future emails. A good reputation means inbox delivery. A bad reputation means spam folder or outright rejection.
Reputation is built on several factors: spam complaint rates (should stay below 0.1%), bounce rates (keep under 2%), engagement rates (opens, clicks, and replies signal legitimacy), sending volume consistency (sudden spikes trigger suspicion), and blacklist status (being listed on major blacklists like Spamhaus is a major red flag).
You have two types of reputation to manage: IP reputation (tied to the IP address you send from) and domain reputation (tied to your sending domain). Domain reputation has become increasingly important as major providers like Gmail shifted toward domain-based filtering. Even with a clean IP, a damaged domain reputation will hurt deliverability.
Monitoring tools include Google Postmaster Tools (free, shows Gmail-specific reputation), Microsoft SNDS (Outlook reputation data), Sender Score by Validity (aggregated reputation score), and your MAP's built-in deliverability dashboards. Check these at least weekly.
Building reputation takes time and discipline. Send to your most engaged segments first, gradually expand to less engaged audiences, never purchase email lists, honor unsubscribes immediately, and maintain consistent sending patterns. Reputation is easy to damage and slow to rebuild.