Your hostname, , and the servername you have set in DirectAdmin, server.host.com, do not match.

If you get the error

where the hostname is blank (note the ", ,"), a reported cause of this is an incorrectly setup /dev/null file.

First make sure the hostname isn't simply blank by checking:

/bin/hostname --fqdn

to ensure you get something displayed.

If you do see something, then the solution is to set it correctly:

rm /dev/null
mknod -m 0666 /dev/null c 1 3

Related:
http://directadmin.com/forum/showthread.php?t=37339

Another confirmed solution for when you get the error message:

hostname: Name or service not known

is to edit your /etc/hosts file, and change:

1.2.3.4 Â your.hostname.com

to be:

1.2.3.4 Â your.hostname.com your

such that you have both the long and short forms of your hostname beside your server IP.

If you do have a hostname in the output, eg:

Your hostname, bob.host.com, and the servername you have set in DirectAdmin, server.host.com, do not match.

it means you just need to update the hostname:

Admin Level -> Admin Settings -> servername

set it, and save. Â You may need to change it to something else, then back again to re-sync it.

Also ensure that you have a full zone for the hostname in your dns:

Admin Level -> DNS Admin -> server.host.com

so that it resolves and has MX records.