SBIN_DIRECTORY

I think exim-0.42 installs all bins in BIN_DIRECTORY. It would be
nice to install utils there, but exim itself in SBIN_DIRECTORY.

I installed everything in /usr/local/bin, which is NFS. That's fine
for exim utils, but the binary itself I'd like to go in /usr/bin.
Otherwise, when NFS server goes away so does exim.