How do you distinguish between technical vs reputation-based bounces?

You get a bounce notification and the first question is always: is this my problem to fix, or just the internet being the internet? That distinction matters more than people realize. A technical bounce and a reputation bounce look similar at first glance, but they call for completely different responses.

Technical bounces are infrastructure problems. The receiving server couldn't be reached, the domain had no MX record, TLS handshake failed, or the connection timed out. None of that is about you or your sending behavior. It's the plumbing. Examples you'd see in the actual bounce message: "host not found", "connection refused", "MX lookup failed", "TLS negotiation error". SMTP codes in the 4.4.x range (network routing) or 5.4.x (routing failures) are classic technical signals.

Reputation bounces are a different story. These happen when the receiving server received your connection just fine, looked at who you are, and decided it didn't want your mail. The bounce message will usually give it away. Phrases like "rejected due to policy", "sender reputation", "blocked by [blocklist name]", or "message identified as spam" tell you the server made a judgment call about you specifically. The 5.7.x code family is the big flag here. 550 5.7.1, 550 5.7.26, that whole neighborhood. When you see 5.7.x, start looking at your sender reputation and authentication setup, not your DNS records.

A quick cheat sheet for reading the signals:

• Technical bounce clues: host not found, MX lookup failed, connection timeout, TLS/certificate error, protocol syntax error. Codes: 4.4.x, 5.4.x, 5.0.x.
• Reputation bounce clues: "blocked", "policy", "spam", a blocklist name or URL in the text, "sender quality", authentication failure language. Codes: 5.7.x.

What you do next depends entirely on which type you're looking at. A technical bounce is often worth retrying. If an address has had multiple technical bounces over time with no pattern, suppress it. A reputation bounce is not a retry situation. It means you need to fix the underlying problem first: check whether you're on a blocklist, review your authentication records, and look at your complaint rate. Retrying into a reputation block just makes things worse.

One thing that trips people up: authentication failure bounces can look technical ("DMARC policy violation", "SPF authentication failed") but they belong in the reputation category. The server didn't have a network problem. It checked your credentials and didn't like them. Fix your SPF, DKIM, or DMARC and the bounces stop. That's behavior, not infrastructure.

If you're seeing a sudden spike in 5.7.x bounces, our free Blocklist Checker is a good first stop. And if you're not sure whether you're dealing with a config problem or a reputation problem, our SOS hotline is free (and we actually read the bounce logs with you).