This is a guide on how to create a correct FQDN hostname on a sever or VPS.
I created this newly here in this section, for easier reference and it's also easier to find.
There is also another method, but the method described here is another method which is also still very usable.
Before you install Directadmin you can already take care the correct hostname is used bij Directadmin.
The domain mydomain.com is used her as example and 192.168.0.1 as example server ip. Ofcourse replace all examples with the domain or hostname your are using. So don't use 192.168.0.1 in the real situation...
Follow the manual I pointed to. It's a step by step manual, should be not too hard.
You can start by changing it in DA itself like in the docs pointed to in the first answer. That should do a lot of good already and is very very easy to do.
If things missing, then please ask in the general section of the forum (not the guides section) and check the guide I pointed to for further reference.