I had the same problem as you. There isn't any real easy way to determine the outside address of your works proxy server, unless you are tight with the IT people and they will supply you with the correct address. What I ended up doing was creating a new ibay and accessing the Ibay from work, then used the httpd access log to find the address that accessed that Ibay. Added that address with a subnet of 255.255.255.255 to prevent any access except from that one IP to the remote access part of the SME Server manager web page. One more thing, make sure that you enable on secure communication when accessing the server manager (ie HTTPS://)
THT,
Darin