Most Common Reply Codes
500 Syntax error, command unrecognized
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
220 <domain> Service ready
221 <domain> Service closing transmission channel
421 <domain> Service not available, closing transmission channel[This may be a reply to any command if the service knows it must shut down]
250 Requested mail action okay, completed
354 Start mail input; end with <CRLF>.<CRLF>
550 Requested action not taken: mailbox unavailable[E.g., mailbox not found, no access]
553 Requested action not taken: mailbox name not allowed[E.g., mailbox syntax incorrect]
The second digit is type of error:
The first digit indicates status: