You can disable local-mail delivery for one  domain name from your shell and follow the instructions..

First go to file
root@admin[~]#vi /etc/localdomains
Then select the domain name give them comment# or remove it and save the file
:wq!

Second step go to file
root@admin[~]#vi /etc/remotedomains
and add the domain name we have comment or removed in the above file
:wq!

Basically when ever we use external mail server at that time we need to remove domains entry from /etc/localdomains file and add it in /etc/remotedomains file