For me, since both my SME Server and the other server are behind the same public IP address (home broadband), I have my router forward a different port number (say 8888) to port 443 on the SME Server. Of course its less aesthetic having to add :8888 to the end of the webmail url but it works. To me, it acceptable since your still using https for webmail.
Not sure if this will work in your situation, but it works for me and hopefully someone else who comes across this thread.
Good Luck