smtp bounce code
550 5.7.1Policy rejection, not a dead mailbox
The server refused you, not the recipient. The mailbox may be perfectly fine. Treating this as a dead address is the most expensive mistake in email verification.
What the server is telling you
Class 5.7.x is authorisation and policy. The server decided not to accept this message, from this sender, right now. Common causes are relay denied, sender reputation, a blocklist hit, or a rule about unauthenticated senders. The recipient is never mentioned in that decision, even when the text says "recipient address rejected".
Next steps
- →Do not suppress the address. There is no evidence it is invalid.
- →Check whether your sending IP or domain is on a blocklist.
- →Confirm SPF, DKIM and DMARC are aligned for your sending domain.
- →If you were probing rather than sending, the server simply refused the probe.
Commonly returned by: Microsoft 365, corporate gateways, most filtered domains
Find these before you send, not after
Paste a list and see which addresses sit on domains that accept no mail, which are one character from a real inbox, and which are on catch-all domains where nothing can tell you anything. Free, no signup, nothing stored.
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