用户的域名解析有问题，mail.域名　没有指向企业邮局的IP，或者mail.域名已经指向IP地址，但MX邮件记录没有解析。

测试方法：

1.ping mail.域名，是否指向正确的企业邮局IP

2.查MX是否正确解析：nslookup –q=mx 域名

C:\>nslookup -q=mx dlsbj.com
Server:  ns.fhnet.cn.net
Address:  202.99.8.1
 
Non-authoritative answer:
dlsbj.com       MX preference = 4, mail exchanger =mail.dlsbj.com
dlsbj.com       nameserver = dns2.hichina.com
dlsbj.com       nameserver = dns1.hichina.com
mail.dlsbj.com  internet address = 218.30.103.167
 
错误的记录
C:\>nslookup -q=mx yldx.com
Server:  ns.fhnet.cn.net
Address:  202.99.8.1
 
Non-authoritative answer:
yldx.com  MX preference = 10, mail exchanger = 218.30.103.173.yldx.com
yldx.com        nameserver = ns000.onlinenic.com