The EPP response codes are described in full in RFC 4930, but the codes used by the Nominet EPP server are replicated here:
| Response code | Response text | Description |
| 1000 | Command completed successfully | Response code for a successfully completed command that is not addressed by any other 1xxx response code. |
| 1001 | Command completed successfully; action pending | Response code for a command that requires offline activity before the operation can be completed. E.g registering a .ltd.uk domain name or requesting a registrar change where the new registrar requires a handshake. |
| 1300 | Command completed successfully; no messages | Response code to a <poll> request command where the server message queue is empty. |
| 1301 | Command completed successfully; ack to dequeue. | Response code to a <poll> request command where a message has been retrieved from the server message queue. |
| 1500 | Command completed successfully; ending session | Response code to a successful <logout> command. |
| 2000 | Unknown command | Response code to a command that is not defined by EPP. |
| 2001 | Command syntax error | Response code to an improperly formed command element. |
| 2002 | Command use error | Response code to a properly formed command element but the command cannot be executed due to a sequencing or context error. E.g. a <logout> command cannot be executed without having first completed a <login> command. |
| 2003 | Required parameter missing | Response code to a command for which a required parameter value has not been provided. |
| 2004 | Parameter value range error | Response code to a request containing a command parameter which is out of the required range. For example this code is returned to a domain create request containing an auto-bill parameter with a value outside of the range 0-182. |
| 2005 | Parameter value syntax error | Response code to a request containing a parameter whose value is improperly formed. For example, this code is returned when a British postcode or domain name is improperly formed. |
| 2100 | Unimplemented protocol version | Response code to a command element specifying a protocol version that is not implemented by the server. This can also be returned if you specify conflicting XML schemas. |
| 2101 | Unimplemented command. | Response code to a valid EPP command element that is not implemented by the server. For example, this code is returned to requests for <contact:create> |
| 2102 | Unimplemented option | Response code to a valid EPP command element that contains a protocol option that is not implemented by the server. |
| 2103 | Unimplemented extension | Response code to a valid EPP command element that contains a protocol command extension that is not implemented by the server. |
| 2104 | Billing failure | Response code to a command to execute a billable operation and the command cannot be completed due to a client billing failure. This is returned by the Nominet EPP server when a <domain:create> command is received and the registrar's credit limit has been breached. |
| 2105 | Object is not eligible for renewal | Response code when a command attempts to renew an object that is not eligible for renewal. The Nominet EPP server returns this code to a request to renew a domain name when it is not passed 6 months before expiry. |
| 2200 | Authentication error | Response code when there is an error when validating client credentials, i.e. the password provided on login is wrong. |
| 2201 | Authorisation error | Response code when there is a client authorisation error when executing a command. For example, when there is a request to transfer a domain name to the NOMINET tag. This response code is also returned by the Nominet EPP server upon a request to query or modify a domain name that is either not registered or is registered to a different tag. This response code is also returned when the usage policy is exceeded. |
| 2302 | Object exists | Response code to a request to create an object that already exists in the database. The Nominet EPP server returns this code to a request to register a domain name that is already registered. |
| 2303 | Object does not exist | Response code to a request to modify or query an object that does not exist. |
| 2304 | Object status prohibits operation | Response code to an operation on an object that cannot be completed due to business practices. For example, the Nominet EPP server returns this code to a request to delete a domain name that has been invoiced |
| 2306 | Parameter policy error | Response code to a command containing a parameter value that is syntactically valid, but semantically invalid. For example, this is returned to a request to register a .com domain name. |
| 2307 | Unimplemented object service | Response code to to a command to operate on an object service that is not supported by the server. |
| 2308 | Data management policy violation | Response code to a command whose execution results in a violation of server data management policies. For example the Nominet EPP server returns this code to a modify request that includes a request to both modify and delete the same field. |
| 2400 | Command failed | This response code is returned when the server is unable to complete a modify, release or renewal command due to database blocking. This error code may be received during the hours of 2 and 4am each day when a large process is run upon our database to update the renewal status of domain names. Once this process has completed the commands can be successfully completed. |
| 2500 | Command failed; server closing connection | This response code is returned when there is an internal server error that is not related to the protocol. The active session is ended and the connection with the client is closed. |
| 2501 | Authentication error; server closing connection | Response code when three invalid login attempts have been received on the connection. The server closes the connection with the client. |