How to interpret ESP bounce classifications?
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You're staring at your bounce report from your Mailchimp dashboard (or wherever you send) and the categories don't make sense. "Hard bounce" and "soft bounce" sound like they should be simple, but they're not. Here's the real deal.
Hard bounces are permanent failures. The address doesn't exist, the domain doesn't exist, the syntax is malformed, or you're blocked by a recipient policy that won't change. These are done. You remove them from your list immediately and never send to them again. Examples: "User unknown," "Domain not found," "Bad address syntax." When you see these, suppress the address.
Soft bounces are temporary. The mailbox is full, the server's temporarily down, or you're being rate-limited and the server wants to retry later. Content filters sometimes trigger soft bounces too. The difference matters because soft bounces usually retry automatically. Your ESP keeps trying for a few days, then gives up if it keeps failing. If you see the same address soft-bouncing repeatedly, it's probably inactive or problematic. Suppress it as if it were hard.
The real skill is reading beyond the label. Not all blocks fit neatly. Some ESPs categorize authentication failures (SPF, DKIM issues) as soft bounces when they're really sender reputation problems. Some blocks look soft but they're really permanent reputational blocks. That's why checking the raw SMTP response code matters. A 550 code is permanent. A 421 or 452 is temporary.
Look for patterns. If you're seeing a lot of "mailbox full" bounces, your list quality is questionable or you're hitting inactive accounts. If a specific domain bounces heavily, that domain's mail server might be misconfigured. If you're seeing authentication-related blocks, check your SPF and DKIM setup. Each pattern tells a story.
What to actually do. Suppress all hard bounces immediately. For soft bounces, let your ESP retry per its policy. Monitor repeated soft bouncers and suppress them after 3-5 failures. Export your bounce reports regularly and look for domain-level or campaign-level patterns. Your bounce data is a diagnostic tool for list quality and authentication problems.
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