SMTP 450 / 451: temporary failures and what to do about them
What 450 4.7.1 ("Try again later", greylisting), 450 4.1.1 ("Recipient address rejected: unverified"), 451 4.3.0 ("Temporary system problem"), 451 4.7.1 ("Message scanning temporarily unavailable") and 451 4.4.0 ("DNS resolution failure") mean, and how each resolves.
What 450/451 means: The server refused this command temporarily but kept the session. Mail queued by a real MTA is retried automatically for hours or days; scripts must implement the retry themselves.
Causes, most likely first — and the fix for each
- 450 4.7.1 "Try again later" / "greylisted": the receiver delays first-time senders to filter bots.
Fix: Retry after 5–15 minutes; the second attempt is accepted. Use a queue, not a single attempt. - 450 4.1.1 / 450 4.7.1 "Recipient address rejected: unverified address": the receiver could not verify the recipient with its backend (sender address verification).
Fix: Retry later; if persistent the recipient mailbox may not exist — check the address. - 451 4.3.0 / 4.3.2 "Temporary system problem" / "Try again later, closing connection": receiver-side outage or overload.
Fix: Retry with backoff; nothing to fix on your side. - 451 4.7.1 "Message scanning temporarily unavailable" (our own servers say this when the content scanner is down): fail-closed anti-spam.
Fix: Retry in a few minutes; the client keeps the message. - 451 4.4.0 / 4.4.3 "DNS resolution failure" / "Temporary lookup failure": the server could not resolve the recipient domain or your reverse DNS.
Fix: Check the recipient domain's MX; make sure your sending IP has valid rDNS. - 451 "Sender address rejected: Domain not found": your From domain has no A/MX record.
Fix: Publish an MX or A record for the sending domain.
How to confirm which one you have
Retry first. If the same 45x persists for more than a day, the "temporary" reason is really configuration — read the text after the code.
Open the SMTP tester Check SPF / DKIM / DMARC
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Questions about 450/451
Standard MTAs retry for 4–5 days with increasing intervals. For application code, 3–5 attempts over an hour, then alert.
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