SMTP code 550 is the most common permanent rejection in email delivery. It means the server will not deliver the message and the sender should not retry. The reasons vary widely: the recipient mailbox does not exist (user unknown), the message was rejected by spam or content filters, the sender's domain or IP is blacklisted, SPF/DKIM/DMARC authentication failed, or a server policy explicitly blocks the sender. The enhanced status code (5.1.1 for bad address, 5.7.1 for policy, etc.) provides more detail. Unlike 450 (temporary), 550 is final — the sending MTA should generate a bounce notification (NDR) to the original sender.
The most common cause — the email address is invalid, misspelled, or the account has been deleted. The enhanced code 5.1.1 specifically indicates 'bad destination mailbox address.' Double-check the address with the recipient.
The receiving server checked the sender's IP against DNS-based blacklists (RBLs) like Spamhaus, Barracuda, or SpamCop and found a match. The 550 response usually includes the name of the blacklist and a URL to check the listing.
The sender's domain has published email authentication records, but the sending server did not pass the checks. SPF fails when the sending IP is not authorized, DKIM fails when the signature does not match, and DMARC failures trigger rejection when the policy is 'reject.'
The message content, subject line, attachment type, or other attribute triggered a server-side filter. Some organizations block messages with certain keywords, executable attachments, or messages exceeding size thresholds.
The server's rejection message usually explains the reason. Look for phrases like 'user unknown,' 'SPF check failed,' 'listed at,' or 'policy violation.' The enhanced status code (5.x.x) provides the category.
Check for typos in the email address. Try contacting the recipient through an alternative channel to confirm their current email address. Accounts may have been renamed or deactivated.
Verify that your domain's SPF record includes the IP address of your sending server, and that DKIM signing is properly configured. Check DMARC alignment as well.
dig TXT example.com +short && dig TXT default._domainkey.example.com +short && dig TXT _dmarc.example.com +shortCheck DNS Records
Look up your sending IP and domain on major blacklists. If listed, follow the blacklist's delisting process. Common blacklists include Spamhaus ZEN, Barracuda, and SpamCop.
Ensure you are sending to the correct mail server. If the domain recently migrated, you may be hitting an old server that no longer hosts the mailbox.
dig MX recipient-domain.com +shortCheck DNS Records
The recipient is not hosted on this server. The server may suggest a forwarding address.
The email address syntax is invalid or the mailbox name violates server naming rules.
The recipient's mailbox is temporarily unavailable. The server will not process the command right now but the sender should retry.
The entire mail transaction has failed. Used as a catch-all permanent rejection, often appearing at connection time.