550 5.7.1 — Message rejected on policy grounds
Your mailbox is misconfigured
SMTP error 550 5.7.1 is a policy refusal: the receiving server accepted the connection, evaluated the message, and declined it. The 5.7.x class covers security and policy, not a bad address, so the usual causes are authentication alignment, sending IP reputation, or content filtering - never a mistyped recipient.
What the server actually says
550 5.7.1 Unfortunately, messages from [203.0.113.42] weren’t sent. Please contact
your Internet service provider. Some bulk mailers require authentication. [AM0P123]What you see
Mail to one provider fails consistently while the same mailbox delivers fine elsewhere. Often appears suddenly after a volume increase on a domain that was never warmed.
Why it happens
- SPF, DKIM, or DMARC is missing, failing, or not aligned with the From domain.
- The sending IP or domain has poor or no reputation - common on brand-new domains sending at volume.
- The domain or IP appears on a blocklist the receiver consults.
- Content triggered a policy filter - spam-like copy, a bad link, or a mismatched display name.
- The receiving tenant has a transport rule explicitly blocking your domain.
How to fix it
- Verify SPF, DKIM and DMARC all pass and align. Check the actual message headers, not just the DNS records. Alignment between the DKIM d= domain, the SPF domain, and the visible From is what DMARC evaluates.
- Read the bracketed identifier in the response. Microsoft returns a code like [AM0P123] and Gmail links a support page. These name the specific policy that fired and are the fastest route to the real cause.
- Check blocklists for the sending IP and domain. If you are on a shared IP you may be paying for a neighbour’s behaviour, which argues for a dedicated sending domain.
- Warm the domain before raising volume. A policy refusal on a new domain is usually just absent reputation. Ramp gradually with genuine engagement before sending campaigns.
What not to do
- Do not suppress the recipient address. 5.7.1 is about you, not them - the same address will accept mail from a sender with better standing.
- Do not classify it as a hard bounce. The 5.x looks permanent, but the condition is your reputation and authentication, both of which are fixable.
- Do not keep retrying at the same volume. Repeated policy refusals reinforce the reputation problem that caused them.
Frequently asked questions
Is 550 5.7.1 permanent?
The individual message is permanently refused, but the cause is not permanent. Fix authentication or reputation and the same recipient starts accepting mail.
Why does only Outlook reject me?
Each provider runs its own policy engine and Microsoft is notably stricter about unauthenticated bulk mail. Passing Gmail is not evidence of passing Outlook.
Does 550 5.7.1 mean I am blacklisted?
Not necessarily. It means a policy declined the message. A blocklist is one possible policy among several - authentication failure and thin reputation are more common.
How MailPilot helps
MailPilot classifies this error automatically on every connected mailbox and tells you which side is at fault, so a throttle is never mistaken for a dead recipient and a credential failure never suppresses a healthy address. It also monitors real inbox placement and checks SPF, DKIM and DMARC on every mailbox, which is where most policy rejections start.
Related errors
- 550 5.1.1 — Recipient address does not exist
- 554 5.7.1 — Blocked by spam filter or blocklist
- 451 4.7.1 — Ratelimit exceeded
Last reviewed 2026-10-11 by the MailPilot team.