Zimbra includes its own web server. It also includes many other services, such as LDAP, IMAP/POP/SMTP, MySQL, Jetty, etc. You would need to make sure that you don't have any of those services running on the server, as they would interfere without reconfiguring a lot of things.
Also, you can change the port of the Zimbra webserver, but you would have to have people access it over the new port, such as
http://mail.server.com:8080
Zimbra is a full suite of programs, and if you have a lot of services running on your server, you may want to install it on a separate system.