After you have set-up the connection with any SMTP server using telnet, you can try different commands, and they’ll be processed and responded to with numeric response codes.
These three-digit coded responses, like 220, may look familiar to Webmasters. The first digit indicates success or failure. 1 indicates a success requiring confirmation, 2 indicates complete success, and 3 indicates success so far, but that more input is expected. 4, the first failure code, is used for failures that may be temporary, such as the mail account existing but being locked; 5 indicates that the failure condition is permanent, such as a syntax error in the command. The second and third digits provide further details, but at the top level, all that matters is that 1xx, 2xx, and 3xx are good, and 4xx and 5xx indicate problems. (For further information on mail response codes, read RFC 1893.)
Tag: codes
