1. 退信'554 5.7.1 This message has been blocked because the return email domain is invalid (failed to obtain DNS)'，发件域名的 DNS 无法解析怎么办？
2. 退信'553 5.7.1 sorry, that domain isn't in my list of allowed rcpthosts (chkuser)'，发不进去对方公司邮箱怎么办？
3. 退信提示'452 4.5.3 Too many recipients'，怎么解决？
4. 退信提示'550 5.4.5 Daily sending quota exceeded'，怎么处理？
5. 退信提示'421 Service temporarily unavailable'，邮件发不出去怎么办？
6. 发邮件被退信提示'550 Ip frequency limited'，怎么解决？
7. 发邮件被退信提示'550 Mail content denied'，怎么办？