版权声明： https://blog.csdn.net/luoshabugui/article/details/82872503
一 文档说明
摘自rfc3501
6.1.2. NOOP Command
Arguments: none
Responses: no specific responses for this command (but see below)
Result:
OK - noop completed
BAD - command unknown or arguments invalid
The NOOP command always succeeds. It does nothing.
Since any command can return a status update as untagged data, the NOOP command can be used as a periodic poll for new messages or message status updates during a period of inactivity (this is the preferred method to do this). The NOOP command can also be used to reset any inactivity autologout timer on the server.
noop命令总是返回成功，但是当服务器有新消息或者消息状态发生改变时，服务器在noop命令响应前会进行响应。
二 举例
C: a002 NOOP
S: a002 OK NOOP completed
. . .
C: a047 NOOP
S: * 22 EXPUNGE // 邮件序号 删除
S: * 23 EXISTS // 收取后，有新邮件到达，邮件总数量，也是当前最大的邮件序号
S: * 3 RECENT // 收取后，有新邮件到达，但没被收取过的数量。（关闭其他邮件客户端时，与EXISTS一起出现）
S: * 14 FETCH (FLAGS (\Seen \Deleted)) // 邮件序号 标签发生改变
S: a047 OK NOOP completed三 讨论
noop是imap的全局命令，既然涉及到邮件序号，那么要处理该类服务器响应，是否应该先进行select命令。