In the same manner, electronic mail gateways are necessary when a message has to travel from one e-mail system to another. Let's start with when you should NOT use the XMail gateway. Other F2 lists download from the master sometime during the middle of the night. Further, these addresses give the most efficient delivery. Unfortunately, in many cases, you just have to 'know'.
Most Internet addresses end with an easily recognizable form, which has a specific meaning.
You can compare it to a telephone situation. Hi, how are you, etc. What's your phone number, we'll get together. Your friend says the number is 491-2345. You, as a CSU campus employee, 'instinctively' know that you dial 12345 from your campus office phone.