Here's how mine is set (using generic terms for the domains etc of course)
Email:
username@domain.tld
Server: zimbrahostname.domain.tld
Username: username
Password: *******
Use SSL: On
That's pretty much it, make sure the iphone can reach port 443 on the zimbra server and you should be good to go. Now, this is using an account on the zimbra server that's in the default domain, even though there are otgher domains present. Try that first to make sure everything is working.
Also, have a look at the log files to see what's happening, try /opt/zimbra/log/audit.log to see login attempts.