How do ESPs classify bounces?

Your ESP sends a campaign, and some emails don't make it. The receiving server fires back a message explaining why. But those raw SMTP responses are messy, inconsistent, and sometimes just plain cryptic. So your ESP translates them into bounce categories you can actually act on.

Here's how most ESPs break it down.

Hard bounces are permanent failures. The address doesn't exist, the domain has been shut down, or the format is invalid. There's no point retrying. Suppress these immediately. Continuing to send to hard-bounced addresses signals to mailbox providers that you don't clean your list, and that hurts your sender reputation fast.

Soft bounces are temporary failures. The mailbox was full, the server was briefly unavailable, or the recipient hit a sending rate limit. Your ESP will typically retry these automatically over a set window (often 24 to 72 hours). If the message still can't be delivered after enough retries, the ESP may promote it to a hard bounce.

Block bounces happen when the receiving server actively rejects the message based on reputation or policy. That could be a blocklist hit, spam content filtering, or a domain policy rejecting your sending IP. These can be temporary or permanent depending on the cause. A block bounce is a signal worth investigating, not just retrying.

Technical bounces cover infrastructure failures. DNS lookup failed, the connection timed out, or TLS negotiation broke down. Usually temporary, often nothing to do with your reputation. Your ESP will retry these.

How ESPs actually figure out which category applies is worth knowing. The raw material is the SMTP response code (5xx for permanent, 4xx for temporary), plus the enhanced status code (the X.Y.Z format), plus whatever the message text says. ESPs often layer in their own pattern-matching logic on top of all that, because receiving servers don't always follow the spec cleanly. Postmark publishes a detailed breakdown of how they classify each bounce type, which is worth a read if you want to understand what's happening under the hood.

The practical takeaway is simple. Hard bounces and persistent block bounces belong on your suppression list. Soft and technical bounces can wait for retry. And if you're seeing a spike in block bounces, that's a reputation problem to solve, not a list problem to suppress away.

So if you're not sure whether your bounce rate is normal for your list size and sending volume, our free Blocklist Checker can tell you whether your sending domain is already flagged somewhere out there.