This is going to be dependent on network that it is being installed on. The quickest way to figure it out is ask the IT department.
Other options if for some reason you can't ask or don't have IT
Look in your Outlook configuration and find the SMTP server address. If it is an IP address you should be able to use it.
If it is something like mail.plcs.net (that isn't one) then inquire with the domain that is state as to which IP address could be used.