What KPIs signal domain reputation issues?
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Domain reputation problems don't usually announce themselves loudly. They show up as gradual shifts in metrics that seem unrelated at first. Here are the signals to watch, roughly in order of urgency.
Sudden drop in inbox placement rate
The clearest signal. If your inbox placement testing tool shows a significant drop (more than 10-15 percentage points) without an obvious cause like a large new segment or infrastructure change, investigate immediately. Check Google Postmaster Tools for domain reputation classification. If it's dropped from High to Medium or Low, you have a reputation problem.
Domain reputation showing Low or Bad in Postmaster Tools
This is Gmail's direct signal. If your domain reputation in Postmaster Tools is "Low" or "Bad," Gmail is actively routing your email away from the inbox for a large percentage of recipients. This is the most authoritative signal you have for Gmail placement and it should trigger immediate investigation.
Spam complaint rate above 0.08%
Rising complaint rate is often the leading indicator before reputation scores drop. Track this weekly. If you're above 0.08%, investigate the source. If you're above 0.1%, act immediately. Above 0.3% is a crisis.
Increase in soft bounces and deferrals
Soft bounces can indicate a reputation problem rather than a recipient problem. If your soft bounce rate is rising while your list hasn't changed significantly, mailbox providers may be rate-limiting or deferring your traffic. Look for 421 or 450 error codes in your bounce logs. These mean "try again later" and often reflect reputation-based throttling.
Open rates dropping for Gmail specifically
So if you can segment your engagement data by email domain and you see Gmail open rates falling while other domains stay stable, that's a domain reputation signal. Gmail users aren't seeing your emails in the inbox even though they're technically delivered.
Appearing on blocklists
Check your sending domain and IP against major blocklists regularly. Our free blocklist checker covers the main ones. A listing on Spamhaus or similar major blocklists is a direct cause of delivery failure and should be treated as urgent.
If you're seeing multiple signals at once, our SOS hotline is free and we've helped a lot of senders work through exactly this kind of situation.
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