How to set localhost name?

[Phil Roan <proan@earthlink.net>]

When I installed debian, I set the local host name on my computer to
local2.  For a long time, everything was fine.  Recently, I'm getting
mail returned to me when I send it to Windows Live Hotmail accounts.
I think the problem is that my mail program (Mutt, with Exim 4 as MTA)
is adding the local host name to some of the headers.  Since the local
host name is different that my ISP name, WLH flags it as suspect mail.

How can I reset (or unset) local host?

Thanks.
-- 
Phil Roan
proan@earthlink.net