smtp;550 cuda_nsu 5.1.1 <[email protected]>... User unknown
The address does not exist, or is no longer active, at this provider. Remove it from your list and check for a typo in the local part or the domain.
Spam filter · 5.x.x permanent failure
Barracuda SMTP 550 Errors: Every Bounce Response Explained
Every SMTP 550 response on record from Barracuda, with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
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.
- Spam filter
- Barracuda
- Code
- 550 (5xx)
- Responses
- 5
- Highest severity
- High
Barracuda SMTP 550 responses
smtp;550 No such user ([email protected])
The address does not exist, or is no longer active, at this provider. Remove it from your list and check for a typo in the local part or the domain.
Status 3#
smtp;550 Blocked
The receiver refused this message outright and will not accept an unchanged retry. The wording above is the only clue to what needs to change, most often the address, authentication, or sending reputation.
Provider documentation
smtp;550 rejecting spoofed message
The receiver rejected the message under a policy rule rather than a content or reputation score. This is usually set by the recipient's mail admin, so ask the recipient to allowlist your sending address or domain.
Status 5#
smtp;550 permanent failure for one or more recipients ([email protected]:blocked)
The receiver refused this message outright and will not accept an unchanged retry. The wording above is the only clue to what needs to change, most often the address, authentication, or sending reputation.
Postmaster & documentation
Barracuda SMTP 550 questions
›What does an SMTP 550 error from Barracuda mean?
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. At Barracuda, it is a permanent rejection, and the specific reason is in the enhanced status code and message text of each response below.
›How do I stop Barracuda from returning SMTP 550?
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check Barracuda's postmaster documentation.