For some reason, the option to edit my above post is not available.
I had received the following email from cpanel:
IMPORTANT: Do not ignore this email.
The hostname (host.hostname.com) resolves to . It should resolve to xx.xxx.xx.xx. Please be sure to correct /etc/hosts as well as the 'A' entry in zone file for the domain.
Some or all of these problems can be caused by /etc/resolv.conf being setup incorrectly. Please check this file if you believe everything else is correct.
You may be able to automaticly correct this problem by using the ' Add an A entry for your hostname ' under ' Dns Functions ' in your Web Host Manager
I knew my "Hosts" and "resolv.conf files were good.
I hadn't read the email correctly, and was wondering whether or not I had to add an "A" entry for host.hostname.com in the DNS Zone for each domain, or to create a separate zone just for the host.
As it turned out, there was no zone for host.hostname.com at all, though there were zones for each domain.
I was able to fix my problem by re-reading the email I received, particularly this part:
You may be able to automaticly correct this problem by using the ' Add an A entry for your hostname ' under ' Dns Functions ' in your Web Host Manager
By going to "DNS Functions/Add an entry for your hostname", the non existent hostname zone was created and thus the problem fixed.
Sorry about deleting the original post. After I posted, I re-read it, and realized that the answer was right in my own question.