Out-of-zone errors

Greetings All,

Have just transferred from bind 4 to 9. All went smoothly (as expected) but 
a couple of domains are giving the following error:

domain.db:34:ignoring out-of-zone data (mail.domain.com)

This is for domains that have their mail servers outside our zone (as per 
the error message). I understand what it is doing, but I know not how to 
fix it. How can I allow such situations to occur?

Cheers,

Lloyd. :-)