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。
| 退信信息 | 错误类型 | 问题原因及解决办法 |
| User not found | 550 | 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 |
| Recipient address rejected: Quota exceeded or service disabled | 550 | |
| Mailbox not found | 550 | |
| This user doesn't have a ×××× account | 554 | |
| The email account that you tried to reach does not exist | 550 | |
| mailbox unavailable | 550 | |
| user not exist | 550 | |
| User unknown in local recipient table | 550 | |
| Invalid User | 550 | |
| mail size or count over quota | 552 | 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 |
| Mailbox has exceeded the limit | 552 | |
| quota exceed | 552 | |
| Mailbox Is Full | 552 | |
| message size ×××× exceeds size limit ×××× of server mail | 552 | 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 |
| mail size or count over quota | 552 | |
| Your message p_w_upload exceeds ×××× limits | 554 | |
| mail rejected by antispam server | 550 | 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 |
| this mail is blocked by kbas system | 553 | |
| sender reject | 553 | |
| Service unavailable | 554 | |
| Your access to this mail system has been rejected due to the sending MTA's poor reputation | 554 | |
| Too many concurrent SMTP connections; please try again later | 421 | |
| System busy now, please come back later | 451 | 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 |
| Service not available - too busy | 421 | |
| Resources temporarily unavailable. Please try again later | 451 |