What happens when a mailbox is temporarily suspended?
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When a mailbox is temporarily suspended, the recipient's mail server returns a 4xx SMTP code. The "4" prefix means "try again later." Your ESP should retry delivery automatically over the next 24-72 hours (depending on their retry policy). If the suspension lifts during that window, the message gets through without any action needed on your end.
The bounce message typically looks like "450 Mailbox temporarily disabled" or "452 Insufficient storage." The key signal is the 4xx code, which tells your system this is a soft bounce, not a permanent failure.
Common reasons a mailbox gets temporarily suspended:
- Security review (suspicious activity flagged by the provider)
- Billing grace period (payment failed, but account not yet permanently closed)
- Storage quota exceeded (inbox full)
- Policy violation under review (content flagged, but not yet permanently banned)
The distinction from permanent closure matters for suppression decisions. A 5xx code (especially 5.1.1 "user unknown" or 5.5.1 "does not exist") means the address is gone and should be suppressed immediately. A 4xx means wait and retry. Most ESPs handle this automatically, but if you're building custom suppression logic, make sure you're not permanently suppressing addresses that only had temporary 4xx failures.
If you consistently see 4xx codes from the same address across multiple campaigns over weeks, that's a signal the suspension may have become permanent even if the code hasn't changed yet. At that point, it's reasonable to suppress proactively.
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