Email alternate addresses are for situations when you expect to receive messages to a specific CU Boulder mail server, even though you don't have an account there.
Registering an alternate email address will allow you to receive messages addressed that way. It doesn't mean you actually have an account on that server, it's only a routing rule that ensures your messages will be delivered.
If this is the only change you plan on making, click on the Save button at the bottom of the page. Then click OK to return to the main menu.