[Rpm-maint] sed a file on rpm install - where in the spec?

[Tom Brown]

Hi

As part of an rpm that i am making during the install i would like to 
sed a file, in this case sendmail.cf. I thought this went in the 
%install however even though the rpm builds it seems this sed is 
executed on the machine thats building the rpm and not on th box that 
the rpm get installed onto.

Can anyone offer any tips as google is not showing me the way.

thanks