[Exim] EECK! I need to create a secondary MX, network proble…

lack of knowledge here.. But what is happening is we have a wireless radio
link that is crapping out on us occasionaly. No big deal, I can get that
looked at, and I can restart it twice a day if I need to. Our
secondary MX (as determined by our ISP, who use someone that is their upstream)
is bouncing mail that is getting sent to them destined for us. It seems
like it might take a bit of time to get that solved, but I can get
a DNS change done pretty quick. I have access to another mail server
running exim, and I can use it temporarily as a MX..
Can someone tell me what config changes to exim on the remote server
I would need to make, and any necessary DNS changes? Any relevant
FAQ references would be great, when I get into the office at least
I have somethig to read, 'cause we aren't getting much email when
the link goes down.. (all last night)
-- 
Derrick MacPherson
Mercury Filmworks