Usermin - localhost is not configured

I've discovered that all my mistakes to date are due to my inexperience with servers and Linux. Most of the answers I need are in the bulletins here ... if I look long enough, I find the answer. So far, Virtualmin's been great.

I do have one little issue yet ... and perhaps I am not looking hard enough for the answer. When I sign into Usermin and go to my Inbox, I see the following in red letters at the top of my page: "An error occurred listing mail in this folder : Failed to lookup IP address for localhost". I also notice that I am unable to reference the Linux machine while logged into it by 'localhost.'

This is most likely a general Linux question, and it perhaps does not belong in here. But if anybody can tell me what I need to do to be able to reference 'localhost' on the machine, I would appreciate it.