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5.x.x permanent failure

SMTP Error Code 550: Meaning, Causes and How to Fix It

SMTP 550 is a permanent failure.

The mailbox is unavailable and the message is rejected for good. This is the catch-all permanent failure, covering everything from an unknown address to an authentication failure to a reputation block, so the wording after the number is what actually tells you the reason.

Class
5xx permanent
Bounce type
Hard bounce
Highest severity
Critical
Responses on record
92
AppleAT&TBarracudaComcastEmail Security.cloudExchange OnlineFireEyeFreeGMXGoogleHeyHostpointImprovMXInfomaniakMail.ruMimecastOrangeOutlookProofpointProtonMailQQRackspaceTrend MicroWeb.deYahoo IncYandexZoho

SMTP 550 responses from email providers

How mailbox providers phrase a 550. Open a provider for the full text, severity and fix for every variation.

SMTP 550 responses from spam filters

How spam-filtering gateways phrase a 550 rejection.

SMTP 550 questions

›Is SMTP 550 a hard bounce or a soft bounce?

It is a permanent failure, so a hard bounce. The receiving server will not accept the same message on a retry, and the address or the sending problem has to be fixed before you resend.

›What causes an SMTP 550 error?

The mailbox is unavailable and the message is rejected for good. This is the catch-all permanent failure, covering everything from an unknown address to an authentication failure to a reputation block, so the wording after the number is what actually tells you the reason. The exact reason varies by receiver, which is why this page lists the specific 550 responses recorded for Apple, AT&T, Barracuda, Comcast, Email Security.cloud, Exchange Online.

›How do I fix an SMTP 550 bounce?

Read the enhanced status code and the message text, not just the number. Wording that names the recipient (user unknown, mailbox unavailable) means correct or remove the address. Wording that names authentication or reputation (SPF, DKIM, DMARC, blocklist) means fix that on the sending side, or clear the listing. Every response on this page has its own guidance.

Other SMTP error codes