为什么80%的码农都做不了架构师？>>> 
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
