What goes into reputation
Authentication — SPF, DKIM, and DMARC must pass and align. Unauthenticated mail is unranked; misaligned mail is suspect.
Bounce rate — high bounce rate signals dirty lists, which correlates strongly with spammers.
Complaint rate — recipients hitting "Report spam" is the strongest negative signal. Gmail flags anything above 0.1%.
Engagement — opens, clicks, replies, "Not spam" marks all build positive reputation. Mail that recipients never engage with degrades it.
Consistency — sending volume that varies wildly looks suspicious. Steady, predictable volume from a warmed sending pool builds reputation faster.
Reputation is per-IP and per-domain
Receivers track reputation by both the sending IP and the sending domain. A fresh IP with great-reputation domain mail often gets a brief boost; a great IP relaying poor-reputation domain mail degrades both.
This is why separating transactional from marketing matters. A marketing send that triggers complaints damages the IP it sent from; if your password resets share the same IP, the next reset email lands in spam too. SNDR routes transactional and broadcast traffic through different MTA pools by default for this reason.
Warming a new sending domain
A brand-new domain has no reputation — receivers throttle volume aggressively until they have observed enough mail to score it. The fix is warmup: ramp sending volume gradually (a few hundred messages on day one, doubling every few days) so receivers can rate-limit you safely while observing engagement.
SNDR's MTA pool handles warmup automatically — fresh sending IPs are weighted lower until they accumulate reputation, then weighted up. The customer-visible effect is that the first week on a new sending domain may see 90-95% inbox rate instead of the usual 99%+ — and recovers on its own.
How to monitor reputation
Gmail Postmaster Tools shows the reputation Gmail computes for your IPs and domains, the volume Gmail received from you, your DMARC pass rate, and your complaint rate. Free, requires DNS TXT verification.
Microsoft SNDS (Smart Network Data Services) shows similar metrics for outlook.com / hotmail.com. Requires per-IP enrollment.
SNDR ships per-recipient-domain inbox rate and per-MTA delivery curves in the dashboard so you see a reputation slide at one ISP before the others learn about it.
Common reputation killers
Sending to a list bought from a third party. Hard bounces and complaints land instantly.
Sending marketing mail from an unwarmed dedicated IP. The volume spike alone will trigger throttling.
Co-mingling transactional and marketing on the same IPs without separation.
Continuing to send to addresses that hard-bounced.
Ignoring complaint feedback loops — receivers expect you to act on the bounce / complaint signals they give you.