554: sending IP or domain on a real-time blocklist
The recipient’s Barracuda gateway refused the connection because your sending IP or domain may be on a real-time blocklist (RBL) it checks.
- Barracuda
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 554
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
554 Service unavailable; client host blocked.
What it means
Barracuda’s Email Security Gateway can check connecting servers against real-time blocklists, and Barracuda documents this reply for a host it blocked that way, noting that the sending domain or IP may be on an RBL. Real bounces often name the list after the words blocked using. It is a permanent rejection. The listing belongs to the list’s operator, so the fix is to find which list it is, deal with the cause and request removal from that list. If the list named is Barracuda’s own reputation list, see the Barracuda Reputation entry instead.
Common causes
- Your sending IP is listed on a public blocklist because of spam traffic from it.
- You share the IP with other senders, or a compromised device on your network sent spam through it.
- A domain in your message or envelope is on a domain blocklist the gateway uses.
How to fix it
- 01Read the full reply for the name of the blocklist.
- 02Check your IP and domain on that list and others with a blacklist checker.
- 03Fix the cause of the listing, then use that list’s removal process.
- 04If one recipient matters while you wait, ask their IT team to allow your IP on their Barracuda gateway.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- SMTP Error Codes, Barracuda Email Security Gateway documentationdocumentation.campus.barracuda.com
Related codes
- 554Transaction failed.The server permanently refused the transaction, often after receiving the message data or straight away when the connection opened.
- 554Blocked by Barracuda Reputation.Barracuda’s own IP reputation list scores your sending server as a spam source, so every Barracuda gateway checking it refuses your mail.
554 at other providers
- 554Permanent rejection (554).Yahoo or AOL rejected the message for good with a 554. Yahoo uses this code for several problems, from failed authentication to unacceptable content, so the wording of the reply tells you which.
- 554Failed authentication.Yahoo or AOL refused the message because it failed the checks that confirm the sending domain, such as DKIM against the domain's policy. Yahoo lists this among its permanent 553 and 554 errors.
- 554Recipient does not exist.The Yahoo or AOL account you wrote to does not exist. The failure is permanent: check the address with the recipient, and remove it from any list.
- 554No reverse DNS (PTR) record.Xfinity refused the connection because your sending IP has no PTR record. Every server sending to Xfinity needs valid reverse DNS, so the owner of the IP must add one.
- 554Rejected for breaking GMX guidelines.GMX refused the message permanently because the connection broke its guidelines. GMX points to its requirements page and asks for the full error message if the problem continues.
- 554IP address on the GMX blocklist.Your sending IP is on a blocklist that GMX manages itself, so its servers refuse the connection. GMX keeps a strict no tolerance policy toward spam.
Further reading
All 11 Barracuda codesEvery code in the wikiPaste your own bounce