If you have iis.mydomain.com and domino.mydomain.com, add those to your virtual domains in SME. Then, in the Hostnames and Address server-manager panel, you can change the visibility and local IP of those subdomains to point to the internal servers.
iss.mydomain.com -> 192.168.1.3
domino.mydomain.com -> 193.168.1.4
Never done it, but it should work.