How do you diagnose reputation loss (Gmail Postmaster, SNDS)?

You've got a nagging feeling something's off with your sending. Your domain reputation might be declining, but you can't just guess. You need to check the actual data. The good news: ISPs publish this data, and it's available to you if you know where to look.

Google Postmaster Tools is your Gmail window. Go to Google Postmaster Tools and connect your domain. You'll see four key metrics. Domain reputation shows as High, Medium, Low, or Bad. IP reputation is rated the same way for your sending IPs. Spam rate tells you the percentage of your mail that recipients mark as spam. And authentication shows your SPF, DKIM, and DMARC pass rates. One caveat: you need minimum volume to see data (roughly 100 messages per day). If you're warming up or testing, you won't see meaningful numbers yet.

Microsoft SNDS (Smart Network Data Services) is the Outlook equivalent. Register your IP addresses at Microsoft SNDS. The data here is more technical but often more honest. You'll see IP reputation scoring, trap hit counts (emails to addresses that don't exist), complaint rates, and filtering disposition (how many messages got marked as junk). Unlike Gmail's reputation labels, SNDS gives you numbers to work with.

Blocklist status matters more than you think. Just because Postmaster Tools shows "High" doesn't mean you're safe. Check Spamhaus, Barracuda, and other major lists using a tool like MXToolbox (they have blocklist monitoring with alerts). Some lists don't publish their criteria publicly, so you might be listed without knowing it. Check both domain and IP blocklists. Getting delisted takes time, so catch listing early.

How to interpret what you're seeing. One bad signal might be temporary noise. But patterns matter. If Postmaster Tools shows reputation decline and SNDS shows rising trap hits and complaints, that's not noise. Cross-reference across sources. Compare against your own historical baseline. Did something change in your sending before reputation dropped? Which campaigns or lists were you sending to when problems started? Are some mailbox providers affected more than others? These questions point to root cause.

Start here: set up Postmaster Tools and SNDS today if you haven't already. Check them weekly. When you spot a decline, that's when you dig into which campaigns caused it.