SMTP応答コード [RFC821]

CodeReason Phrase
211System status, or system help reply
214Help message
220<domain> Service ready
221<domain> Service closing transmission channel
250Requested mail action okay, completed
251User not local; will forward to <forward-path>
354Start mail input; end with <CRLF>.<CRLF>
421<domain> Service not available,
450Requested mail action not taken: mailbox unavailable
451Requested action aborted: local error in processing
452Requested action not taken: insufficient system storage
500Syntax error, command unrecognized
501Syntax error in parameters or arguments
502Command not implemented
503Bad sequence of commands
504Command parameter not implemented
550Requested action not taken: mailbox unavailable
551User not local; please try <forward-path>
552Requested mail action aborted: exceeded storage allocation
553Requested action not taken: mailbox name not allowed
554Transaction failed