产生退信的原因有多种多样，下面根据您接收到退信中的关键字，来查找一下可能的原因。
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退信信息
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错误类型
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问题原因及解决办法
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User not found
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550
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收件人地址有错误 收件人地址错误或处于不可用状态，请检查收件人拼写或其它方式联系收件人 |
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Recipient address rejected: Quota exceeded or service disabled
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550
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Mailbox not found
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550
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This user doesn’t have a ×××× account
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554
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The email account that you tried to reach does not exist
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550
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mailbox unavailable
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550
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user not exist
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550
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User unknown in local recipient table
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550
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Invalid User
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550
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mail size or count over quota
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552
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收件人邮箱空间不足 收件箱或邮件超空间限制，请用其它方式联系收件人或稍后再试 |
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Mailbox has exceeded the limit
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552
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quota exceed
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552
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Mailbox Is Full
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552
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message size ×××× exceeds size limit ×××× of server mail
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552
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邮件大小超出限制邮件大小、附件大小超出收件服务器限制，减小邮件大小后再试
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mail size or count over quota
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552
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Your message attachment exceeds ×××× limits
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554
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mail rejected by antispam server
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550
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邮件被对方服务器拒绝 邮件被收件方服务器判为了垃圾邮件，向邮件管理员反馈或用其它方式联系收件人 |
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this mail is blocked by kbas system
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553
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sender reject
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553
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Service unavailable
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554
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Your access to this mail system has been rejected due to the sending MTA’s poor reputation
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554
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Too many concurrent SMTP connections; please try again later
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421
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System busy now, please come back later
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451
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邮件服务器服务异常 邮件服务器服务异常(或反垃圾策略限制原因)，稍后再试或向管理员反馈 |
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Service not available – too busy
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421
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Resources temporarily unavailable. Please try again later
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451
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