This may be due to the means of authentication being used -- by the STMP server you are sending to and/or the authentication options selected in your Entourage setup. Mail servers commonly establish restrictions based on authentication to restrict who can send mail through them as a means of preventing spam.
Cable companies for example, and possibly your campus network, often restrict the sending of Mail to those computers that have a local connection to their network based on the IP address they are using -- i.e. where you plugged in or which wireless adapter you are connected to. Alternatively, many others require your mail client to present your user name and password to authenticate... or possibly to authenticate only in those cases when you are not connected locally.
So... to get an iron clad answer, you need to find out from the Mail server administrators what type of authentication is required. This information is often listed on "help" web pages describing how to set up your mail client... or call them if you can. Alternatively, check your Entourage setup to see how it is contacting the server for outgoing mail. Try changing the authentication method to use Password authentication if it is not already. Ultimately, the mail administrator for your Mail server is likely your best help resource.
Hope this helps.
