SMTP code 551 means the server recognizes the recipient address but the user's mailbox is not hosted locally. The server may include a forwarding address in the response (e.g., '551 User not local; please try <[email protected]>'). This was more common in early SMTP when mail relaying was widespread. In modern email, 551 is rare because most servers either handle the address themselves or reject it outright with 550. When it does appear, it usually indicates a server migration where the old server still knows about the user but no longer hosts their mail.
The recipient's mail has been moved to a new server but the old server still recognizes the address. Instead of silently accepting and forwarding, it returns 551 with the new destination. This is common during organizational email migrations.
The server has an explicit forwarding rule that returns 551 instead of transparently relaying. This is a security-conscious approach — rather than acting as an open relay, the server tells the sender to connect to the correct destination directly.
The domain's MX records direct mail to a server that does not host the recipient's mailbox. This can happen after infrastructure changes when MX records are not updated to reflect the new mail server.
If the 551 response includes a forwarding address, resend the message to that address. The server is explicitly telling you where the recipient's mail now lives.
Check if the domain's MX records point to the correct server. If MX records are outdated, mail will be delivered to the wrong server which then returns 551.
dig MX recipient-domain.com +shortCheck DNS Records
If no forwarding address is provided, contact the recipient through another channel to get their current email address, or contact the domain's postmaster for routing information.
If MX records were recently updated, verify the changes have propagated. Your sending server may be caching the old records.
Check Propagation