That’s an odd response code. The ESMTP error codes typically follow a structured format of three numerical fields separated by dots, where the first digit indicates the class of the error (e.g., 2 for success, 4 for a transient negative, and 5 for a permanent negative). There are no codes that start with a 6.
Additionally, that text is not found in the application source code, so it is a message being returned by the server you have configured. If your provider offers support, you may want to ask them, as you may not find an answer here.