What’s the difference between 550 and 554 errors?
Still have a question, spotted an error, or have a better explanation or a source we should cite?
Both 550 and 554 are permanent failures. Your message isn't going to be retried. But they're signaling different problems, and the fix is different too.
550 is recipient-level rejection. The most common flavour is "User unknown" or "Mailbox unavailable," meaning the email address doesn't exist or the account is disabled. When you see 550 errors in your bounce reports, it usually means your list contains invalid addresses. Remove them immediately. They won't start working later. Continuing to send to hard-bounced addresses hurts your sender reputation.
554 is a policy-level rejection. The address itself might be valid, but something about the sender, the message, or the sending IP has been refused by the receiving server's policies. Common reasons include: the sending IP or domain is on a blocklist, authentication failures like SPF or DMARC rejection, content filtering triggered by specific message characteristics, or explicit sender blocks set up by the receiving organization.
The full text after the error code tells you more. "554 5.7.1 Message rejected as spam by Content Filtering" points to content. "554 5.7.1 Service unavailable; Client host [IP] blocked" points to IP reputation. Read the full error string and work backward from there.
If you're seeing persistent 554 errors to a specific provider, check your IP against major blocklists with our free Blocklist Checker, and verify your SPF and DMARC records are correctly configured.
Contributors
Who worked on this answer
Every name links to their profile. Every company links to their site. Real people, real accountability.
