A member post a email to the maillist.Then get a strange response

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subject:Your message to Oz_maillist awaits moderator approval

content:

Your mail to 'Oz_maillist' with the subject

    =?gb2312?B?suLK1NTT1r4=?=

Is being held until the list moderator can review it for approval.

The reason it is being held:

    Message has implicit destination

Either the message will get posted to the list, or you will receive
notification of the moderator's decision.

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What is the reason?