SMTP 响应

简单邮件传输协议 (SMTP) 响应从服务器发送到客户端。 SMTP 中的每个响应都以三位数代码开头，后面可能是附加的文本信息。响应中的前导数字表示响应的类别。以下是不同类别的回复：

1. Positive completion reply
2. Positive Intermediate reply
3. Transient Negative Completion reply 
4. Permanent Negative Completion reply 

这些解释如下：

1. 积极完成回复 –
   此回复表示成功完成操作。之后可以发起新的请求。
2. 积极的中级答复 –
   此回复是指接受我们收到的命令，但请求的操作正处于暂停状态，并且还在等待收到进一步的信息。
3. 瞬态否定完成回复 –
   此回复是指未接受命令且未发生请求的操作。在这种情况下，错误条件是暂时的，并且可能会再次请求操作。

4. 永久否定完成回复 –
   此回复表示未接受命令且未发生请求的操作。

下表列出了各种 SMTP 响应：

Category	Code	Description
Positive Completion Reply	211	System status and system help reply.
	214	Help message.
	220	service ready.
	221	service closing transmission channel.
	250	Requested mail action okay, completed.
	251	User not local; the message will be forwarded to .
Positive Intermediate Reply	354	Start mail input; end with .
Transient Negative Completion Reply	421	service not available, losing transmission channel.
	450	Requested mail action not taken, mailbox not available ( e.g. Mailbox busy ).
	451	Requested action aborted; local error in processing.
	452	Requested action not taken; insufficient system storage system.
Permanent Negative Completion Reply	500	Syntax error, command unrecognized.
	501	Syntax error in parameters or arguments.
	502	Command not implemented.
	503	Bad sequence of commands.
	504	Command parameter not implemented.
	550	The requested action is not taken due to the unavailability of mailbox ( e.g. Mailbox not found, no access ).
	551	User not local; please try .
	552	Requested mail action aborted; exceeded storage allocation.
	553	The requested action is not taken because the mailbox name does not allowed it (e.g., Mailbox syntax incorrect.).
	554	Transaction failed.