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
