That is how you are supposed to have your hosts file.
Here's what you should do:
-Ditch the virtual domain.
-Just create a new/2nd domain.com in the admin console gui > domains section.
-Don't delete the host.domain.com yet, as there are admin reports/alerts slated to be delivered there.
Though you can certainly delete it if you'd like after you move the admin/spam/ham and update all the references.
You'll see what I mean if you:
su zimbra
zmprov gacf
Most just leave spam/ham alone and just create an alias of
admin@domain.com in
admin@host.domain.com.
You can also rename the spam/ham accounts (like if you wanted to remove the random numbers) - again be sure to update your zmlocalconfig.