If all new domains are going to have their mail hosted remotely, and you wish to have this option set by default you can do so with the domain_create_post.sh script.
Create:
/usr/local/directadmin/scripts/custom/domain_create_post.sh
In it, add the code
#!/bin/sh
FILE=/etc/virtual/domains
cp -f $FILE $FILE.backup
grep -v -e "^$domain\$" $FILE > $FILE.tmp
mv -f $FILE.tmp $FILE
chmod 644 $FILE
chown mail:mail $FILE
exit 0;
chmod 755 /usr/local/directadmin/scripts/custom/domain_create_post.sh
Any newly domains created after this point should not be in the /etc/virtual/domains.
Don't forget to copy:
/usr/local/directadmin/data/templates/dns_mx.conf
to the custom folder (in that same directory) and edit the custom/dns_mx.conf for the correct default vaules.