How do “delivery errors” correlate with reputation scores?

You open Gmail Postmaster Tools and see delivery errors climbing. Your first question: is this a technical glitch or is Gmail telling you it doesn't trust you? The answer lives in the error code, and in whether your reputation score is moving at the same time.

Let's back up for a second. Reputation score in Google Workspace Postmaster Tools reflects how Gmail feels about your domain or IP based on signals like spam complaint rates, engagement history, and authentication passing. It's not a number you can look up directly. It's a band: High, Medium, Low, or Bad.

Delivery errors, on the other hand, are SMTP-level rejections. These are the moments when Gmail's server turns your email away before it even gets a chance to land. The code it returns tells you a lot.

• 421 errors are temporary deferrals. Gmail is saying "not right now." This is almost always throttling, which Gmail does when your sending reputation has slipped but hasn't fully tanked. Your messages may eventually get through if reputation improves, but sending more volume while you're getting 421s usually makes things worse.
• 550 errors are permanent rejections. Gmail is saying "no, full stop." A 550 with a policy explanation (like a 550-5.7.1 code) is a direct signal that your domain or IP is blocked due to reputation. This is the clearest confirmation that the problem isn't technical.
• Other error codes like DNS resolution failures or connection timeouts tend to point at infrastructure problems: a misconfigured sending server, an IP with no reverse DNS, or an authentication setup that's broken. These can happen even with a clean reputation, so they're worth ruling out first before blaming your reputation score.

The diagnostic move is to look at both graphs side by side. If your reputation score drops and delivery errors rise at the same moment, that's a reputation-driven rejection pattern. Gmail saw something it didn't like (a spike in complaints, a sudden volume increase, an authentication failure) and started rejecting traffic. If errors spike while reputation holds steady, go hunting for a technical cause first.

One thing the current answer got right: errors compound the problem. Every rejection that Gmail logs is more data. If you keep sending into a wall of 421s or 550s without fixing the root cause, you're not just stuck, you're actively digging the hole deeper.

A few things to check when errors and reputation are both dropping at once:

• Has your domain reputation graph moved from Medium to Low or Bad in the same timeframe?
• Are your SPF, DKIM, and DMARC records all passing? An authentication failure can trigger both a drop in reputation and an uptick in policy-based rejections.
• Did you send to a stale or unvalidated segment recently? A batch of bad addresses will inflate both bounces and complaint signals.
• Is the error affecting all recipients or just Gmail? If other providers are also rejecting mail, the problem is more likely infrastructure. If it's Gmail-specific, reputation is the more likely culprit.

If you're seeing 550-5.7.1 rejections and your reputation graph shows Low or Bad, that's not ambiguous. That's Gmail telling you directly that trust has broken down, and the fix is a sending pause, a list clean, and a gradual warm-up to rebuild the signal. You can check your authentication setup with our free Email Header Analyzer to see if something upstream is contributing. And if you're staring at a dashboard full of errors and not sure what to fix first, the SOS hotline is free and we actually pick up.