在做邮件营销的推广的过程中，邮件退信率是一个重要的指标。
邮件退信率会影响到发邮件的域名，IP的信誉值，退信率越高域名IP的信誉值越低，邮件将直接进入垃圾箱。
根据EmailDiamond的数据分析，电子邮件的退信率上升的主要原因在于：
1、收件人邮箱地址不存在。
2、收件人邮箱错误
3、邮箱域名不存在
4、MX记录不存在
5、邮件列表存在陷阱邮件地址
6、收件人的地址为一次性的地址
常见的退信代码：
521 Recipient address rejected
550 5.1.1 https://support.google.com/mail/?p=NoSuchUser
550 5.1.1 https://support.google.com/mail/answer/6596
550 Invalid User
550 Mailbox not found
550 user not exist
550 user not exist
550 User not found
554 delivery error: dd Sorry your message to $EMAIL cannot be delivered. This account has been disabled or discontinued
554 delivery error: dd This user doesn’t have a $DOMAIN account ($EMAIL)
421 4.7.0 [xx] Message from ($IP) temporarily deferred
421 4.7.0 [xx] Message temporarily deferred
421 4.7.0 https://support.google.com/mail/answer/188131
450 Requested action not taken
451 DT:SPM
550 DT:SPM
550 SC-001
521 5.2.1 : AOL will not accept delivery of this message
550 5.7.1 Message rejected as spam by Content Filtering
550 Mail content denied
550 Suspected spam
554 Message not allowed
423 Dns resolve error
524 Host not found by dns resolve
526 No data by dns resolve
EmailDiamond总结了以下几种避免邮件退信技巧
1、确保邮箱地址正确
2、过滤掉陷阱邮箱
3、确保邮箱地址经过验证和清洗
4、优化发信邮箱的域名信誉值
5、避免发信IP进入反垃圾邮件黑名单
