These settings below should work.
The reverse proxy access the https part of Zimbra (if you're using a self-signed certificate you'll have to download and save as /etc/apache2/ssl/zimbra.crt).
Code:
<VirtualHost *:443>
ServerName reverse.domain.tld
SSLEngine on
SSLCipherSuite ALL:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL
SSLCertificateFile /etc/apache2/ssl/reverse.domain.tld.crt
SSLCertificateKeyFile /etc/apache2/ssl/reverse.domain.tld.key
SSLProxyEngine on
SSLProxyCACertificateFile /etc/apache2/ssl/zimbra.crt
RequestHeader set Front-End-Https On
ProxyRequests On
ProxyPreserveHost On
ProxyVia full
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
ProxyPass / https://internal-zimbra-server.domain/
ProxyPassReverse / https://internal-zimbra-server.domain/
</VirtualHost>