The virtual Server that runs this Joomla site and Zimbra has 3 public IP addreses. As Zimbra binds to port 80 for its HTTP web client I needed to force Zimbra to only bind to one of these IPs so that Apache running Joomla could bind to another. This allows http://www.all-our-email.com to connect to Joomla and http://all-our-email.com to connect to Zimbra.

• edit /opt/zimbra/mailboxd/etc/jetty.xml.in
      add <Set name="Host">your-ip-address</Set> to the HTTP connector
• edit /opt/zimbra/mailboxd/etc/zimbra.web.xml.in
      uncomment zimbra.soap.url, change localhost:7070 to your-ip-address
• edit /opt/zimbra/mailboxd/etc/zimbraAdmin.web.xml.in
      uncomment zimbra.soap.url, change localhost:7070 to your-ip-address
• su - zimbra
• zmcontrol stop
• zmcontrol start

This applies to Zimbra 5.