Oke that is a quite different question indeed.
They can use the name without the @domain name. I thought they changed this to be active by default, it might not.
You can fix it like this.
First add this line to your /usr/local/directadmin/conf/directadmin.conf
file:
system_user_to_virtual_passwd=1
then restart DA
service directadmin restart
If it is still not working then use this command for all users:
echo "action=rewrite&value=email_passwd" >> /usr/local/directadmin/data/task.queue
or this command for the specific user:
echo "action=rewrite&value=email_passwd&user=fred" >> /usr/local/directadmin/data/task.queue
This can take up a minute or 2 and then it should work.