Smtp Return values

We recommend that you check the smtp protocol. we recommend that you check the smtp protocol:
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* Email service return code meaning
* 500 format error, command unrecognized (this error also includes command line too long)
* 501 incorrect parameter format
* 502 commands cannot be implemented
* 503 incorrect command sequence
* 504 command parameters cannot be implemented
* 211 system status or system help response
* 214 help information
* 220 service ready
* 221 disable the transmission channel
* 421 if the service is not ready, disable the transmission channel (this response can be used as a response to any command when it must be disabled)
* 250 the required email operation is completed
* 251 users are not local and will forward
* 450 the email operation is incomplete and the email address is unavailable (for example, the email address is busy)
* 550 the email operation is not completed and the email address is unavailable (for example, the email address is not found or cannot be accessed)
* 451 discard required operations; errors occurred during handling
* 551 users are not local. please try
* 452 The System Storage is insufficient and the required operations are not performed
* 552 excessive storage allocation, required operations not performed
* 553 the mailbox name is unavailable and the required operation is not performed (for example, the mailbox format is incorrect)
* 354 start email input and end.
* 554 operation failed
* 535 user verification failed
* 235 user verification successful
* 334 wait for the user to enter verification information