421  Service not available, closing transmission channel (This may be a reply to any command if the service knows it must shut down)
450  Requested mail action not taken: mailbox unavailable (E.g., mailbox busy)
451  Requested action aborted: local error in processing
452  Requested action not taken: insufficient system storage
500  Syntax error, command unrecognized (This may include errors such as command line too long)
501  Syntax error in parameters or arguments
502  Command not implemented
503  Bad sequence of commands
504  Command parameter not implemented
550  Requested action not taken: mailbox unavailable (E.g., mailbox not found, no access)
551  User not local; please try
552  Requested mail action aborted: exceeded storage allocation
553  Requested action not taken: mailbox name not allowed (E.g., mailbox syntax incorrect)
554  Transaction failedThe other codes that provide you with helpful information about what’s happening with your messages are:
211  System status, or system help reply
214  Help message (Information on how to use the receiver or the meaning of a particular non-standard command; this reply is useful
only to the human user)
220  Service ready
221  Service closing transmission channel
250  Requested mail action okay, completed
251  User not local; will forward to
354  Start mail input; end with . (a dot)
‘*************************  
  ‘*   邮件服务返回代码含义  
  ‘*   500   格式错误，命令不可识别（此错误也包括命令行过长）  
  ‘*   501   参数格式错误  
  ‘*   502   命令不可实现  
  ‘*   503   错误的命令序列  
  ‘*   504   命令参数不可实现  
  ‘*   211   系统状态或系统帮助响应  
  ‘*   214   帮助信息  
  ‘*   220     服务就绪  
  ‘*   221     服务关闭传输信道  
  ‘*   421     服务未就绪，关闭传输信道（当必须关闭时，此应答可以作为对任何命令的响应）  
  ‘*   250   要求的邮件操作完成  
  ‘*   251   用户非本地，将转发向  
  ‘*   450   要求的邮件操作未完成，邮箱不可用（例如，邮箱忙）  
  ‘*   550   要求的邮件操作未完成，邮箱不可用（例如，邮箱未找到，或不可访问）  
  ‘*   451   放弃要求的操作；处理过程中出错  
  ‘*   551   用户非本地，请尝试  
  ‘*   452   系统存储不足，要求的操作未执行  
  ‘*   552   过量的存储分配，要求的操作未执行  
  ‘*   553   邮箱名不可用，要求的操作未执行（例如邮箱格式错误）  
  ‘*   354   开始邮件输入，以.结束  
  ‘*   554   操作失败  
  ‘*   535   用户验证失败  
  ‘*   235   用户验证成功  
  ‘*   334   等待用户输入验证信息