I had a test setup for the past few weeks. Now I want to change the hostname, it was "webmail.domain.com" and it should be "mail.domain.com". I have used the wiki to do this, with the command:
/opt/zimbra/libexec/zmsetservername mail.domain.com
Without any error the command executed, I have added the new hostname in the hosts file, set the server name with:
hostname mail.domain.com
And rebooted the server.

Now, when I log on to the administration page I get two servers in my status screen (mail + webmail). But in the screen servers I only see the new hostname. And when I send an e-mail to a local account I still see the line:
webmail postfix/lmtp...
In my maillog. Besides that, the server tries to connect to my old mail.domain.com server with an old IP address to deliver the e-mail. But when I ping to the name mail.domain.com it returns the new IP address.

Totally confused....