It works fine for me with just https set for a connection. Have you set-up your DNS correctly? How did you change the mode? It would help, when describing a problem, to list the actual version of Zimbra you're using and which operating system, use 'zmcontrol -v' and update your forum profile with that information.
Check your DNS with the following (run on the Zimbra server):
and post the output.
host `hostname` <-- use backticks not single quotes
dig yourdomain.com any
dig yourdomain.com mx