depends on how you installed it ... did you install kde with konstruct, or did you install the sources by hand or did you install a package (rpm,deb)?
[1] for konstruct:
go into the directory that lies one level above kmail (konstruct/kde/kdepim/work), type
make buildclean
patch -p0 < file.patch
make install
[2] for sources:
go into the directory that lies one level above kmail, type
make clean
patch -p0 < file.patch
./configure
make && make install