No Mail Exchange records have been configured for your domain. This means mail servers on the internet cannot find your mail server for your email address.
To correct this problem you should login to your registrar's domain management tool and configure at least one MX record and set its target to an Address record which points to your mail servers IP address
You can check your Mail Exchanger records yourself from the command line on most operating systems. The most common command for doing this is the utility nslookup.
To check your MX record status simply run nslookup and type:
Press return and then type in your domain name (Note: if you're address is email@example.com then only type in example.com)
If you have done this correctly you should receive a response similar to this one:
Default Server: dns.server.local
> set type=mx
example.com MX preference = 10, mail exchanger = mail1.example.com
example.com MX preference = 20, mail exchanger = mail2.example.com
example.com MX preference = 30, mail exchanger = mail3.example.com
example.com MX preference = 40, mail exchanger = mail4.example.com