Traditional Bounce Codes

421<domain> Service not available, closing transmission channel
450Requested mail action not taken: mailbox unavailable (e.g., mailbox busy)
451Requested action aborted: error in processing
452Requested action not taken: insufficient system storage
499Unable to connect to MX servers (e.g., wrong email address or destination email server is down)
500The server could not recognize the command due to a syntax error.
501A syntax error was encountered in command arguments.
502This command is not implemented.
503The server has encountered a bad sequence of commands.
504A command parameter is not implemented.
550User’s mailbox was unavailable (such as not found)
551The recipient is not local to the server.
552The action was aborted due to exceeded storage allocation.
553The command was aborted because the mailbox name is invalid.
554The transaction failed for some unstated reason.