Most deliverability metrics you can measure yourself are proxies. Postmaster Tools is different: it reports Gmail’s own view of your sending domain and IPs, including the spam-complaint rate — the one number you can’t see any other way, because you never find out when a recipient hits “Report spam.” If you send any meaningful volume to Gmail, this is the first place to look when placement slips.
Verify your domain first
Sign in at postmaster.google.com with a Google account, add your sending domain (the domain in your From: address), and Google gives you a TXT record to publish in DNS:
Domain-verification TXT record from Postmaster Tools:
Type Host / Name Value
TXT @ (example.com) google-site-verification=AbC123...xyz
Add it exactly as Postmaster Tools shows it, then click
Verify. If you already verified the domain in Google Search
Console, Postmaster Tools recognises it automatically.Add the TXT record at your DNS host, wait for it to propagate, and click Verify. Two notes: the domain you verify must match the domain your mail is authenticated with, and dashboards only populate once you send enough volume — Google needs a threshold of daily traffic (in the hundreds-plus range) before it shows data, to protect recipient privacy. Low-volume senders may see “not enough data,” which is expected.
Reading the dashboards
Once data flows, Postmaster Tools gives you several charts. Here’s what each one tells you and how to act on it.
Domain & IP Reputation
Gmail rates both your domain and each sending IP on a four-level scale: Bad, Low, Medium, High. High means your mail is trusted and lands in the inbox; Bad means most of it goes to spam or is rejected. Domain reputation is the more durable signal and the one to watch closely — a slide from High to Medium is an early warning to investigate before placement collapses. If reputation is Low or Bad, the usual causes are complaints, poor list quality or inconsistent volume; work through how to check and repair your sender reputation.
Spam Rate
This is the most important number in the whole tool: the percentage of your delivered mail that Gmail users marked as spam. Keep it under 0.3% at all times, and aim for below 0.1%. The 0.3% figure is Gmail’s stated bulk-sender threshold — cross it and you risk throttling and spam-foldering; sustained spikes cause real damage. Spam rate is user-reported, so it’s driven almost entirely by relevance and consent: only mail people who asked for it, make one-click unsubscribe trivial, and honour it fast. A rising spam rate is the clearest signal to tighten your list and your opt-in.
Authentication
Separate pass-rate charts for SPF, DKIM and DMARC show the share of your traffic that authenticated successfully. These should sit at or near 100%. Anything materially below means some of your mail is misconfigured or being sent by a source that isn’t set up correctly — a dip here often precedes a reputation drop. Cross-check with the Email Health Check and the DMARC Checker, and if SPF passes but DMARC lags, it’s an alignment problem — see why SPF passes but DMARC fails.
Encryption (TLS)
Shows the percentage of your mail sent to Gmail over an encrypted TLS connection. This should be at or near 100% — modern senders and receivers negotiate TLS automatically. A low number points to a transport misconfiguration on your sending infrastructure worth fixing, since some receivers now expect encryption in transit.
Delivery Errors
Breaks down the share of your mail Gmail rejected or temporarily failed, categorised by reason — rate-limiting, reputation, authentication, content, and so on. This is your fastest diagnostic when volume suddenly drops: the category tells you why Gmail is refusing mail. Rate-limiting errors mean you’re sending faster than your reputation supports (slow down or warm up); reputation errors point back to the reputation and spam-rate dashboards; authentication errors send you to your SPF/DKIM/DMARC setup.
Turn it into a routine
Postmaster Tools is a monitoring surface, not a one-time check. Bookmark it and glance at reputation and spam rate on every significant send, and after any change to your sending setup. Pair it with a few habits that keep the underlying signals healthy: verify lists before sending with the Email Verifier, keep your bounce rate low, check blocklists with the Email Blacklist Check, and run periodic full checks with the Email Health Check. Postmaster Tools covers Gmail; for Outlook and Hotmail, add Microsoft SNDS alongside it, and turn on DMARC monitoring to catch any unauthorised source sending as your domain.
Sources
Set up and read the dashboards at Google Postmaster Tools; Google’s guidance for bulk senders, including the 0.3% spam-rate threshold, is published in the Gmail sender guidelines. For Outlook and Hotmail, the equivalent per-IP data is Microsoft SNDS. The authentication whose pass rates the tool reports is defined in the DMARC specification, RFC 7489.