What are Postmaster Tools?

Google Postmaster Tools is a free service from Google that shows how Gmail treats your email. It's the closest thing to seeing inside Gmail's filtering decisions.

What it shows:

Domain reputation: High, Medium, Low, or Bad. This directly affects inbox placement at Gmail.

IP reputation: The same scale for your sending IPs.

Spam rate: Percentage of emails marked as spam by recipients. Critical metric.

Authentication results: SPF, DKIM, and DMARC pass rates.

Encryption: TLS usage for your email connections.

Delivery errors: Why Gmail rejected messages.

How to set it up:

Verify domain ownership (DNS TXT record)
Wait for data to populate (requires sufficient volume)
Check regularly for changes

Limitations:

Only shows Gmail data (not Outlook, Yahoo, etc.)
Requires minimum volume for data to appear
Some metrics lag by a day or more
Doesn't show individual email details

If Gmail were a major shipping port, Postmaster Tools would be the harbor master's log book. It won't tell you everything, but it reveals whether your vessels are welcome or watched.