You may be ok with one IP. Does ActiveSync address the server by domain name or by IP?
By domain name, but I'd still need a reverse proxy for this to work wouldn't I since ports 80 and 443 need to be forwarded to different internal servers depending on the requested URL?
I was hoping it might be possible to avoid needing a reverse proxy by having multiple public IPs.
Well, you can't easily get more static IPs, so a reverse proxy seems to be the answer. It's easy to put an Nginx server in your DMZ then proxy_pass requests through to private internal IPs based on the domain name or domain + path requested.