What is “IP temporarily blocked”?
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An "IP temporarily blocked" message means a receiving mail server has decided to stop accepting mail from your IP address for now. It's a short-term throttle, not a permanent listing. Your ESP should retry delivery automatically, and if the block lifts before the retry window closes, the mail gets through without any action on your end.
You'll typically see this as a 421 or 450 SMTP error code, often with wording like "temporarily deferred," "try again later," or "connection refused." These are transient errors, not hard bounces.
Common triggers:
- Sudden volume spike (looks spammy even if it isn't)
- Elevated spam complaint rate
- High bounce rate from invalid addresses in a recent send
- Sending patterns that trip automated filters (burst sending, unusual timing)
The difference from permanent blocklisting: A temporary block is set by the receiving server's own local policy and expires on its own timetable. A blocklist listing is a third-party database entry that any server can query independently. You can be temporarily blocked without being on any external blocklist, and vice versa.
Most temporary blocks lift within 24-48 hours if you stop triggering them. If you're seeing repeated temporary blocks to the same provider, that's a signal your IP reputation with them is degraded. For Gmail, check Google Postmaster Tools. For Outlook and Hotmail, use Microsoft SNDS (Smart Network Data Services). Both are free and show you what the mailbox provider actually sees.
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