Hi Bandit,
If you change the server hostname it shouldn't directly affect qmail.
Technically you shouldn't need to make any changes to qmail unless you want qmail to operate as a different server name too.
In your control directory /var/qmail/control/ you'll want to check these files:
- defaultdomain
- locals
- me
- plusdomain
- rcpthosts
There may be more files you need to check depending on your configuration.
You'll also need to restart qmail for it to pick up changes to some files.
Don't forget to check any related DNS entries too.