How to track mailbox-full or temporary deferrals?
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Your ESP's bounce log speaks in SMTP codes. Two of them cover most temporary deferral situations, and knowing what they mean helps you decide what to do with each address.
452 4.2.2 (Mailbox full): The subscriber's inbox is genuinely over quota. This is usually safe to retry and keep on your list short-term. But if you see the same address returning this code across multiple campaigns, they're not actively reading email. That's a list hygiene signal, not just a delivery blip.
421 4.7.0 or similar (Try again later): The receiving server is rate-limiting or temporarily unavailable. This is about the server, not the individual subscriber. If you see this code across many addresses at one domain, the provider is throttling your IP.
Where to find these codes: most ESP dashboards roll all soft bounces into a single category without distinguishing the cause. To see actual SMTP codes, you'll need to export the bounce log or look at campaign-level delivery detail reports, depending on your ESP.
What to do with the patterns you find:
- Mailbox-full codes on the same address after 3+ campaigns: move to a monitor segment, stop sending for 30-60 days
- 421 codes clustering at one domain: reduce your send rate to that domain and check if your sending IP is listed on any blocklists
Our free blocklist checker is a quick way to see if widespread deferral from specific domains is linked to a listing. And for more on what deferrals do to your delivery metrics more broadly, that's worth a read alongside this.
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