I keep getting these messages in my zimbra.log as well as emailed to zimbra@domainname.com.

postqueue: warning: My hostname XXXX is not a fully qualified name - set myhostname or mydomain in /opt/zimbra/postfix-2.2.3/conf/main.cf
Is it safe to set mydomain manually in main.cf? Will this break the multiple domains that I'm hosting on the zimbra server?

Thanks ---