Google Postmaster Tools: the sender's window into Gmail

Google Postmaster Tools (GPT) is a free dashboard at postmaster.google.com that shows how Gmail evaluates your sending domain and IP addresses. If you send email to Gmail recipients and need to understand why messages land in spam or get rejected, GPT is the first place to look.

Setting up

Sign in with a Google account, add the domain you send from, and verify ownership by adding a TXT record to your DNS. Verification usually completes within minutes. Data begins appearing once your sending volume to Gmail addresses reaches a few hundred messages per day.

What GPT reports

Spam rate. The percentage of your messages that Gmail users marked as spam. Google requires bulk senders to keep this below 0.1%. At 0.3%, blocking begins. Data is shown per day.

Domain reputation. A four-tier rating: High, Medium, Low, Bad. High means Gmail trusts your domain and delivers to the inbox. Bad means most of your mail goes to spam or gets rejected.

IP reputation. The same four-tier scale, applied to each sending IP. On shared IPs, the score reflects all senders on that IP, not just you.

Authentication. Pass rates for SPF, DKIM, and DMARC. All three should be near 100%. A DKIM pass rate of 80% means some of your messages are sent from servers that do not sign with your DKIM key.

Encryption. The share of messages transmitted over TLS. Gmail flags unencrypted messages with a warning icon.

Delivery errors. The percentage of messages rejected by Gmail, with reason codes: rate limit exceeded, suspected spam, bad reputation. This section explains why mail is not getting through.

Common problems and fixes

• Spam rate above 0.1%. Check whether you are sending to inactive subscribers. Confirm that the unsubscribe link works and is easy to find. Process FBL complaints promptly.
• Domain reputation Low or Bad. Pause the affected segment. Scrub the list for spam traps. Send only to your most engaged subscribers until the score recovers.
• DKIM pass rate below 100%. Some messages leave without a valid signature. Audit every sending source, including your ESP, CRM, and transactional mail servers, and confirm each one signs with your domain key.
• Delivery errors: rate limit. Reduce sending speed. Set up throttling and warm up any new IP gradually, adding volume over days, not hours.

Reading the data

Data appears with a 1-2 day lag. Look at trends over a week, not individual days. A single spike in spam rate is not a crisis; a sustained rise over several days demands action. A domain reputation drop from High to Medium calls for immediate investigation.

Limitations

GPT covers Gmail and Google Workspace only. For Outlook, use Microsoft SNDS. For Mail.ru, use Postmaster Mail.ru. GPT does not reveal individual complainant addresses (unlike a traditional FBL). All data is aggregated and anonymous.