有關GMAIL - Google

請問我剛寄信時 GMAIL回傳這個給我

內容是

Delivery to the following recipient failed permanently:

[email protected]

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the recipient
domain. We recommend contacting the other email provider for further
information about the cause of this error. The error that the other server
returned was: 550 550 5.7.1 <[email protected]>: Recipient address
rejected: User is unreachable Online_Check_Fail RCPT_TO 550 <ntus (state 14).

是信件無法寄出嗎?

這是對方的問題還是自己的問題?

感謝

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