What’s the difference between temporary and persistent deferrals?
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Both types look the same in your bounce log: a 4xx response that means the message wasn't delivered yet. The difference is what happens over time.
Temporary deferrals are exactly what they sound like. The receiving server is temporarily unavailable, briefly overloaded, or applying a short delay for greylisting. Your sending infrastructure retries, the server accepts the message on the next attempt, and the email delivers. Most of the soft bounces you see in daily sending fall into this category. They resolve without any action on your part.
Persistent deferrals keep failing across multiple retry attempts over hours or days. The server keeps returning 4xx codes but never accepts the message. At some point your sending infrastructure gives up (most ESPs have a retry window of 24-72 hours before treating an unresolved deferral as a failed delivery).
Persistent deferrals are a signal that something is wrong on your end. The most common causes:
- Sender reputation: The receiving provider has enough negative signals about your IP or domain to severely throttle your delivery, short of an outright block.
- Rate limiting: You're sending to a specific provider faster than they're willing to accept, and you're not slowing down between retries.
- Filtering on the content or subject line causing repeated deferral rather than an immediate block.
The key distinction operationally: temporary deferrals need no list action. Persistent deferrals need investigation. If specific addresses or entire provider domains are showing persistent 4xx failures across multiple campaigns, check your sender reputation at that provider and review your sending behavior to understand what's triggering the extended throttle.
Not sure if your deferrals are resolving or persisting? Your ESP's delivery report should show whether retries succeeded or whether the message ultimately failed. Look for "deferred" vs "failed" in your delivery status column. If you can't find that data, ask us and we'll help you find it in your specific platform.
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