I do have a webserver/emailserver behind my ZS router, although i dont use 443, and i simply put a rule in the virtual server
ppp0/ANY TCP 80 [local server address]:80
and we have no issues connecting internal/external.
also point the ZS DNS to ur webserver in DHCP section as first DNS and ZS as second DNS and if u have a domain make sure ZS knows about it in the same section.
hope this helps
JC