421 Service not available 리턴이 됩니다.
어떤 이유인지 알 수 있나요?
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NOTICE: Delivery Failure.

This Delivery Status Notification is sent from MTA...                  Your delivery to the following address has been failed. Please refer to the below for details.                 ------------------------------------------------------

Delivery failed: ****@**** Connected to 173.xxx.xxx.xxx but greeting failed. Remote host said[Response Message]: 421 Service not available, closing transmission channel I'm not going to try again; this message has been in the queue too long.