I only use normal http for now, but this is how I did it:
For me personally, the hosted DNS point mail.domain.nl to the IP address the apache server runs behind.
Allow from all
ProxyPass / http://mail.domain.nl/
ProxyPassReverse / http://mail.domain.nl/
Internally, the DNS server point mail.domain.nl to the internal ip address (10.0.0.2) of the zimbra server.
I've kept the names the same for the outside world as the inside world. This to make sure zimbra does not point to links that do not exist outside the lan.