Can't send out email from horde with default email address

I have installed horde in my DA.

And it works fine with all the email addresses that i have created.

But if i login with my username, i can't send out email from horde.

For eg.
my domain: kacang.com
my username: kacang

if i login horde with my username "kacang", then i can't send out any email.

It works fine with squirrelmail.

Check the default hostname horde is using to send email with. It has to match the hostname set in DA.