"101": "The server is unable to connect"
"111": "Connection refused or inability to open an SMTP stream"
"200": "System status message or help reply"
"214": "A response to the HELP command"
"220": "The server is ready"
"221": "The server is closing its transmission channel"
"250": "Success"
"251": "User not local will forward"
"252": "Cannot verify the user, but it will try to deliver the message anyway"
"354": "Start mail input"
"420": "Timeout connection problem"
"421": "Service is unavailable due to a connection problem"
"422": "The recipient's mailbox has exceeded its storage limit"
"431": "Not enough space on the disk"
"432": "Recipient's incoming mail queue has been stopped"
"441": "The recipient's server is not responding"
"442": "The connection was dropped during the transmission"
"446": "The maximum hop count was exceeded for the message"
"447": "Message timed out because of issues concerning the incoming server"
"449": "Routing error"
"450": "User's mailbox is unavailable"
"451": "Aborted – Local error in processing"
"452": "Too many emails sent or too many recipients"
"471": "An error of your mail server"
"500": "Syntax error"
"501": "Syntax error in parameters or arguments"
"503": "Bad sequence of commands, or requires authentication"
"504": "Command parameter is not implemented"
"510": "Bad email address"
"511": "Bad email address"
"512": "Host server for the recipient's domain name cannot be found in DNS"
"513": "Address type is incorrect"
"523": "associated with encryption in RFC 5248"
"530": "Authentication problem"
"541": "The recipient address rejected your message"
"550": "Non-existent email address"
"551": "User not local or invalid address – relay denied"
"552": "Exceeded storage allocation"
"553": "Mailbox name invalid"
"554": "Transaction has failed"